Title: Certain well-factored categories
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00098432/00001
 Material Information
Title: Certain well-factored categories
Physical Description: vi, 68 leaves : ill. ; 28 cm.
Language: English
Creator: Maxwell, Stephen Jackson, 1945-
Publisher: s.n.
Place of Publication: Gainesville FL
Publication Date: 1970
Copyright Date: 1970
Subject: Categories (Mathematics)   ( lcsh )
Algebra, Homological   ( lcsh )
Genre: bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaf 67.
General Note: Manuscript copy.
General Note: Vita.
 Record Information
Bibliographic ID: UF00098432
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000570681
notis - ACZ7662
oclc - 13719940


This item has the following downloads:

certainwellfacto00maxwrich ( PDF )

Full Text







To ry rnoth rand to the imoory of rdy father


The author wouldd lik. to express his sincere gratitude

to his director Profossor U. E. Clar-k, for his mathematical

assistance his prince; ni, his Cen3riosity in giving aid

on numerous cccasions. The author would also like to express

his appreciation to his roommate, IMr. Burrow Brooks, for

the loan of his typewr-iter



AC O:'O-LEDG... '7 -




Chaptc '



. .. .*.a . 0 oe0Q aeoe* **e a c

....e.oe ......a o*. .aea.. a oa6a 6

I. PRELIMIN~L RIES ........ c .e.............

II. TWO EiM-BDD:I: TH O' -?! ..... .... .. .


BIBLIOGRAPHY .. .. o .. .. .. ... ... .... .... .

BIOGRAPHiCAL .'CH ....... ...... c r c c .... o














DIAGRAMi. (5')









oLOJO*leeOe~eooee+ eo0e GcGY

..9 ......&o&aoe.a0 C L& .c .a a a 0

a 0 o..o0 a 0 e0o 6 & 0 0 6 a a 0
. . . ..C.. a c6& a60 &000a ca

.. ..0. . . . . . 0

. 6 0....0 0c 0 ... 0 .04 ... & 00 0 0 0c c 0

Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy



Stephen Jackson Maixwell

August, 1970

Chairman: Professor W. E. Clark
Major Department: Mathematics

A new kind of category, called a well-factored category,

is defined. This is a generalization of the category of

all modules over all rings, the category of all fibre bundles

with fixed fibre, and the category of all topological acts.

Two structure theorems are proved for well-factored

categories. One is an embedding into a "product" category

and the other is an embedding into the category of all

left actions in a multiplicative category without unit.

These embeddings preserve the factorization of morphisms

given in the definition of a well-factored category.

A sufficient condition for a category of left actions

to be a variety, as defined by Herrlich, is given. As a

corollary, it is noted that the category of all modules

over all rings is a variety.


Let P be a 'rin3 and r be the category of all left

ER-r.oul Gs. Th. study of the category .. is already well

develop o and farilier. Thi. author set out to study the

casteory of all noDiules over all rineg, denoted 71. Thus,

each Ti is a subcatecory of 1. and morphisms in R are
R, PI R_
semilinoir transfozrations (f,g); i.e., f is a ring homo-

norophisr, g ris a group homoLorphism~ and g(rm) = f(r)g(m),

for each r and u. The author then noted that numerous

cat:eories sach a- the category of all topological acts,

the category of all compact acts, the category of all fibre

bundles vith fi'sd fibre' the csatgory of all ncnoi ds eating

on sets, and the cateory of all blodules over all pairs

of rin-s posses prop'ioLiC similar to those of RM Mos
imIpcrntly, iLn al.l those- ccatgcories, -e nay uniquely factor

each morph srI as the compo-stion of two mo.rphiss belonging

to corr'esponlinS clrses of orphlsis.s. ,Roughly speaking,

for V1, -: ha v-e "odu-le" rmoro.piss of the form (.1,) and

''ring'" nmorphi sms of the for-m (f,1). These e::e-plos and

the gen;ralizing concept of a w-ell-factored category are

present Ln Chapter To

In C.L ":tcxr IIl ; o prove' tio, structure theorems for

l -factoreI cct;ories W show that a well-factored

cateCCr y b .-.d .. a. very nice way into the "oduct"

of three categories. Then we show that a ;ell-factored

category may be embedded in a very nice ;.ay as a subcategory

of the category of all left action in a rultipiicotive

category -without unit,

Herrlich i introJuceJ in [1] an aniciaatically defined

varietal cateo ory whichh generalized the varieties of Lawvere

and Linton. We give in Chapter III a sufficient condition

for the category of all left actions in a multiplicative

category without unit to be a varietal category in the sense

of Herrlich. A corollary to this result is that the cate-

gory of all nodules over all rings is a variety. le have

a generalization of this condition and list some corollaries

to the generlt'Trion but, due to the pressures of tinme

we onit the proof. We hope to publish the complete results


An important part of the proof for our condition given

in Chapter III is that the forgetful functor U:Lact(T)---

EnsxEns has a left adjoint. We were partially motivated

by the construction of a left adjoint for U when N is Ens

with the product functor and the usual associativity trans-

fornation. This construction was given in C2].



We will use the 2notaLion of [-3] throughout, unless

stated otherwise. Also, fa-iliari~ y on the part of the

reader )ith Chapters I. II, and IV of [3] will be assumed

In this chapter, wie will state the definition of a well-

factored category, give some examples of woll-factored

categories, can ,'tatc some kno-m results which are not

assured to be background knowledge for the r-eadero

(1.1), Rrji o For the following definitions, let

C be a category and let Pr and P2 be two :classes" of sub-

catego:.ies of C such that each object C in C is an object

in ecly on b of and in exactly one nrber of P2

Let f be a morphism in C. Then f is said to be a P,-r0orphism

if an6 only if f is a norphisia in some category i.n P~

i=!~,2 Note then that the composition of Pr-morphisms

is a P. -oorphisr i1;,2

(1.2). Notaticon. Ab wrill denote the category of all

abslian groups. R uill cdeoote the category of all rings

which do not paces arly have an identity, R' wil. denote

the category of all rings having an identity and all identity-

preservi.-g rig ho.onorr-phic.s. Enl. is the category of

all sets,

(1.3). Din;iTti~c, .t : =-U:IorA and U i:orA or
1 2

vice versa. Lot f and g be in N, say f:A-3 and g:A'-^BA.

We say 'f and reS 'd f' l!i by M.-morphisms if and

only if there exist sequen e Si, Sn of N-morphisms

sach that each S. is not the empty sequel nce the elements

of each Si need not be coach :able, S. = (f) S = (g),

and, for each i = 1 ... n-1, one of the following (or one

of the following with S. and S, interxchaiged) occurs:

(1) S = (g 9 ., 1' -il kk- ,.

Si+1 = n "* g:-1 kd-:' kl* *0$1
where 'g indicates gk has been deleted from the sequence.

Also, ck is an identity such that if N is the collection

of all P. morphi~ then cod Gk-l dom gk, and dom ongk

are all objects in the some f -subcategory of C, i=1,2 and

i $ j If k = n, then we have the above conditions for
k and k, If k :- 1, then we have the above conditions

for k* and k-+1

(2) SS i * k+1 *

Si+1 i (, k+1 k,. *- 1
where fofl is defird a i equal to g

(3) S = (k"i 9"'"f +1 r k"***

S ;, :': ( 9 k fi'91 9Sk '- )
and there cxist -. C';;lphiss i and 1' so that moo = gk+lm;

ieoe, the follow .~-ri dinZ':caj! co prLItos :

E '-+W t

(-i4). .fi.n 1on, We say th.at C is c. factored by

sn P2 if oi I the follow, th'- conditions hold.


(1) If fl and f2 are ~-,-orphi ss as indicated by
subscript, then flof2 is a Pi-morphism implies f~ is an

identity, where i j, i = 1,2, and j =- 1,2.

(2) If fl is grid connected to g, by Pr2-morphisms
an1 f2 is rid colneted to G2 by Ps-norphisms, where
A ,---J.- and A- -2-, then f2o 1 = g2og"1

(3) For each orphlism f in C, there is a unique
factorization f = hog such that g is a rF-morphism and
1. -
h is a _2-morpphism.

We will denote this situation by C= P X f .

(1.5). Examonpe. Let S be a monoid. We may consider
S as a category in the usual manner. If S is rell-factored

by P1 and P2, then the cardlnality of Fi is one. i = 1,2.

Also, S is isomorphic to P ~ where 1 is identified

with its menlir, for i = 1,2, and X denotes uonoid product.

Proof, We must put off the proof that this example is

true until the next chpter.

(1.6). ,voile. Lot G tbe a group. Then G may be con-
sidered, in the v -'3.1 manner, as a category. Suppose G

is well factored by FP and ,. Then the cardinality of

Pi is one, for i -.= 1,2 Identifying Pi with its member,
for i = 1,2, :we have G = P1Xr where X denotes the direct

product of groups.

Pr.cf. We rmst put' off the proof that this example
is true until the next chantr;i

(1.7). r 1i t:.t'o." Lt.a C C and C be categories.
Define the cati-oryi .' 2 '' as foll.owsc the object

class of C X C x C is equal to the object class, of
-1 -2 3
C 22 x C 3 Also, a "orphlsn in C x C x C is of the
form (fGg)(C ,,C2,C )---(C C. ,), where f:C --> is a
morphlsm in C, and 0:C---->3 is a morphism in C. It i.s
-- 2 2
clear -that C1 x 2C x C is indeed a cabe oryo
1 -) 2 -,-
(1.8). PT-o-ton. Let C C and C be categories.
Let CI be an object in C1 Let P ( CC) be the subcategory
of C x C.2 C given by:

Obq(C1) = {(C1,C2C ) Ci 6 Ob, i = 2,3i
(1,G) c c
and Morf1(CI) = (0c,&C2 C3): >(C1,CC3) g.:C-- >C'
is a C2-morphisn and 1 = 10c. Let i 1(Cl1) Cl O-bC-
Define P2 ana:ogously; i.e., define by restricting the
second coordin-ate. Then any suboategoryAof x x 3
in which (3) of 1.4 holds and such that (1,;1)(AP A2,A )-
(A1A.A2,A3) is an A-.morphism, implies A 3 is =ell-fao-
tored by the restrictions of ar and t to A. We call this
the standard ',ell- factorization of A,
Proof. Let Abes such a category. It is clear that
each object in A is an object in exactly one member of P
and in exactly one member of rP. First, we prove that (1)
of 1.4 holds in A. Lret (1p<) : (B3.B2, E) --(B ,A2,) and
( ,1)'(C1,B2,C3)--~(Bi B2,Bt ) Then (1,) o(P,1) = (~-; )
implies c = I, Then by hypothesis B = A. Thus, (1,<<) is
an identity. Similarly, (1,0)() (l,,) implies (p,i )
is an identity. Thus, (1) of 1.4 holds. Suppose

A(:t -) (2f ,l)
(AA, A,)------- ->( AI, E ,B (CI9BD2C') aad

(1, ) (g ,1)
(A,,A )) >(AD2,B')- 2 >(C1,B2,C3) where
(l,f1) and (1,G1) a:.' grid connected by P2-morphisns and

(f2,1) and (g2,1) ere grid connected by Fr-morphsiss. It
suffices to prove If = g- and f g2o e will. prove f = g

since the proof that f2 g2 is analogous. We use the
sequences of P- nohis. front 1.3, here S1 (1) and
Sn = (gl). wHe ill prove the following inductively: if
Si = (Sni.0. ,! then gni = (1,hni )" ^1 = )(1hl)

hn is_ "_is defined and if S1 (g ,.,g gn+ =
ni +1 1 ni+1
(~,hn,.i) '' 1 = (,hlh),9 then hnio **oh1 = he "'..oh1,
for i = 1,...,n-1. Suppose i = 1. By 1.3, we have three
possible cases.

(1) S1 = ((1~1)) and S2 = ((1,1),(,f)) or S2
((1,f2)(1,1)). Then cod(1,fl) and dom(l,l) are in the
same P2-c at gory implies cod(!,fl) dom(1,1) = cod(1,1).
Thus, the inductive statcnent is true here for i = 1.
(2) S1 ((1, f)) e S = ((1,f), (.1f)), where
(1if)o(1,) = (1. f). -Ti-, the inductive statement for
i = 1 is obviously true here.
(3) S ((1f )) and S2 = ((l,h)) where there exist

P,2-morphi.sms (m, ) arnd (i,1I) such that (m' ,1)o(l,h) =
(l,fi)o(mn1). Thus, h = fl, so the inductive statement
for i = 1 is true here.
Going from Si to Si+1 is quite similar to the above.
Thus, fl = g1" Similarly, f2 g Hence, (f2,1)o(,f) =

(2)(1G).Thu, is well-,factored.
(1.9), -: ,I 1-. ne ,7 eh cibit a product category

such that condition (3) of 1,4 does not hold in all of its
subcategories, Let C be the full subcategory of Ens x E ns
Ens x Enis 0 whose objects are pairs (X,Y) of sets X and

Y which have the saz_ cardi-ality. Lot 2 have the standard
well-fastori o ion6 Let inc :1}--~{ ,2L be the inclusion

function. Thse-n (i1ncinc);:( 1 ,{1})-->(1,2>,1, 2>) is a
C-morphism. -o;erer, it is easy to see that this has no

P1- -2 factorization,
(1.10). xiU:;pe We now, exhibit a category C x C x C
-1 2 x3
which is not vlell-fatored in the standard way. Let C =

C = C 3 Ens. Then it is easy to see that the morphism
(1 ,1) (1, {1~3 3 1})-> 1(1, ( ,1i 2>) has infinitely many
P~ faotorizations. Note also that the condition (1,1):
(C1PC2,C3)-->(C,C2pC ) is a morphism implies C C fails.
(1.11). 1ole. Let A be a category. Let ) be the
diagram schere given by 1-->2. h-,[.. ,A] is the category
of all diagr is in A over. For convince of notation,
we will denote t:A1--->A2 as (A1,A2t), Let C = C A
and let C be the discrete category such that ObC = HIorA.
3 3 =
Then[C ,A] is a subcategory of C x C x C Suppose (1,1):

(A~A2 ,t)--->( ,A2 ). Then lot = to1 so t = t'. Suppose
(fg):(A1,A2 --- t) Then (1 g):(A,A2,t) >

(AAtof) and (fAA t) a nd (f,g) =

(f,) o(i,g). Furthermore, this is the uniouc F factor-
ization of (f,g). Thus, by 1.8,5C ,A is well-fa-tored.
Special cases of interest are:
(1) If =-R, then l[.,A] is denoted R nd is well-

(2) If A = 1, then L[ -l is denoted by 1R1.
(3) If A = Top, the cs-egory of all topological spaces,

then iEA is denoted as ?-n, the category of all bundles.

(1.12). Exgagle, Let RI be the category of all modules
over all rings. That is, an object in M is a triple (R,-,p)
where R is a rin, i is an abelian group, and p:R x M-i->
is a function so that (R,M,p) is a left P-module. A morphism

in R taking (R,!,p) into (R',iIQ,p') is a pair (f,g) so
that f is a rin-g morphisn taiRing R into B', g is a group

morphism taking I into ,'I and, for each r in R and m in i,

we have that g(rm) = f(r)g(m). Lot be the subcategory

of RJ consisting of all E-modules and so that a morphisr

(f,g) in Ri is a morphism in Ri if and only if f = 1R.
Whenever possible, (R,I,p) is denoted by (R3,.;). Let R.

be the subcategory of RL consisting of all modules having

carrier equal to 1. A morphism (fzg) in 71 is a mor-hisn

in 1:! ilf ani only if g = 1I. Let P1 be thB collection of
all H such that E is a ring. Let P be the collection

of all such that N is on anablian group. Then Ni = 0 x Po

Pr-oof Since P, and Pf are the standard well-factori-

zat-ion, -.:e may apply 1.8. Suppose (1,1): (;I,p)--(RM;p').
In the dcain; rm = p(r,n). In the codomain: rt'' = p'(r,m).

Thus, p'(rmn) = rm = (r)*1(m) = (rm) = l(p(r,n)) = p(r,n).

This says p' = p. Suppose (fg): (R,,;p)--( ,.(p')

Then (1,g) : (,I:,p)--(RI ,p (fx1)) ai (f,1):r, (fxl))

-->(R,H' p') and (f,1)o(ig) = (f,g). Moreover, this is
the un'i.q~ P-P fctorizaic Then by 1.8 -we have that

R- 1 2
(1.13). .lo. Let be the category of all bi-
modules over all rings. Am. object (R,i,S,p19p2) in i

such that R and i are rings, II is an abelian group, pl:

Rxn--)I9, p2:.Xs-->::, (R;,MIl) is a left R-module, (M,S,p2)

is a right S-modale, and, for each r in R and m in MI1 and
s in S, we have (rm)s = r(ms). A morphism in R_ taking

(R,I,S) into (R1Il,S') is a triple (f,g,h) so that f:_R-->P

is a ring morphism, h:S-- 3 is a ring m norphism, arnd g: I.-->iM

is a group morphisn so that, for each r in R and m in M

and s in S, g(rm) = f(r)g(m) and g(ms) = g(m)h(s). Whenever

possible, we denote (R,',S,pl,p2) by (R,MS).

We define P.-subcategories analogously to P -subcate-

gories in 1.12 by holding the rings constant. We define

P2-_subcategories analogously to P2-subcategories in 1.12
by holdi-ng the abelian group constant. Then 1, = P x 2.

Proof. This proof is analogous to the proof of 1.12.

We chance. notation from (RllS,pi,p2) to ((RS),iG,(pip2))

and from (f,g,h) to ((fh),g).
(1.14). F amr.ple Let rm be the full subcategory of

P' determined by the objects of the form (R,;, R). Then
the ell-factorization of Rp given in 1.13 induces a well-

factorization of .

Proof. This is a straightforw-ard application of 1.8.

(1.15). .--"--~1es. We define the categories IM R M 1'
and I, -, =
and R1~! analogously to respectively I by

replacing R with R1. We also obtain :i 1 -factorizations

of these categories in an analogous fashion. Finally,

Sand I are wiell-factcorod, where no:. ;e have right nodules

rather than left modules.

Proof. It is clear that these examples are valid.

(1.16). E,:;-e. Lt C bt the category of all topo-

logical acts. An object in C is a triple (S,Xp:S-X--6 X)

such that S is a Hausdorff topological senigroup, X is a

Hausdorff space, p is continuous and gives the multipli-

cation, and, s and t in S and x in X, (sb)x = s(tx). We

denote (S,X,p) by (S,X) Thenever possible. A morphism in

C taking (S,X) into (S',X') is a pair (f,g) such that f:S-->S

is continuous and preserves the semicroup multiplication.

g:X---X is coontinuou.s, and., for each s in S and x in X,

g(sx) = f(s)g(x).

C is a subco.t : of C x C2 x C There C is the
-1 -2 3 C1
category of all topological senigroups, C2 is the category

of all Eausdorff spaces, an.d C3 is the discrete category

having as objects al:l p:S X--CX so that p is continuous

and andnd X are Hausdorff spaces. Then C is well-factored

by the standard well-fctorization.

Proof. This is an easy application of 1,8.

(I.7?). ',, plc Let Co:np C be the category of all

compact topological acts. That is, Comp C is the full

subcategory of C deteriincd by the objects (SX) in C such

that S and. X are compact. Then Comp C is well-factored

by the factcri ation induced from C.

Proof. ',We not tht the i the nduced factor ization is just

the standard

the proof is


G-spaoe is a

The image of

(1) For

(2) For

i ell-factorization of Corp C. With this fact,

an easy al?.cation of 1.8.

EBan2DJ. If C is a topological group, a right

topolloical space X along with a map X>3-->X.

(x,s) is denoted xs. We require the following:

each x in X and' s and sl in G, x(ss') = (xs)se.

each x in X; .71 = x, .where 1 is the identity

in G.

A G-space is called cffecti-ve provided xs = x implies

s = 1. Let X = 'xxs)EXXX x s in X and s is in C->,

where X is an effective G-space. There is a function t:

X"-->3 defined by xt(x,x') = x' and -hich is called the

transl.tion functio0.- A G-space X is called -oril.cipal

provided X is an effective G-space with a continuous trans-

lation function. A rJirr-r-. G-bund-le is a bundle (X,p,B),

where X is a principal G spa e. A morphism (u,f):(Xp,3)-->

(X',p',BA) between two principal G-bundles is a prnpQ p~al

no irphi-s o provided u:X--- is a G-Module homomorpnhisn and

is continuous. Let PDBn be the category of principal bundles

and principal rorphisSs. Let Pauni be the subcategory of

PBan obtained by holding B in (X~p,B) constant and PEunX

be the subcategory obtained by holding X in (Xp,B) constant.

Then morphisns have identities in the "constant" coordinate.

Let P. be the classs of all P:BnX sand let P2 be the "class"

of all PEn Then PEun = 1 x F2.

?Proof Since 1i and r are the factorization induced
on PEnn 1 the w-ell--factori zation of Dan given in 1.11,

it suffices to prove that condition (3) of 1.4 holds.
Suppose (u,f):(X,p,E--B)--(X' p',). Then (1,f):(X,p,B)--

(X,fop,B') and (u,1) :(: fo p,C )-- (X',p',B') and (X, fp, B)
is an object in P-.n,, The rcest is clear.

(1. 19). :---rl. For fu'rter details about the cate-'
gory of fibre handles ai, about 1.18, the reader should
consult [43 To elements x and x' in a G-sTace X are

said to be G-equivalent if and only if there exists s in

G such that xs = x'. This is an equivalence relation, so

we may foin the quotient space, denoted X/G,

Let c = (X.p,B) be a principal G-bundle and let F be

a left G-spa~ce. The equation (x,y)s = (xss-ly) defines

a r-ight G-space structure on XxF. Let Xp denote the quotient

space (Xe?)/G and let pF:Xp---B be the factoriza.tion of

XXF Pr'l-X by the canonical map X;F-->X. Thus,
for (xy) in X?, pp((xy)G) = p(x). Then (Xpp",B), de-

notbed d[FC, is the fib .e inldle over B with fib-re F (vie-wed

as a G-space) ,sn -as ccia tid :1 ,i2 nc3Dgl b'-Idle dc A fibre

s-orohiam from dWC] to d'[F] is a bundle morYphism of the
form (uFpf):d[.]--'d'[F3, where (uif):d- is a principal
bundle norphisi an d up is obtained as follows: u:X--)>X

induces a G--orphism u;:I:XX;--;X; F aend u? is the induced

quotient !ap from (WXP)/G to (X'.)/G. Let iun[F] denote

the category of all fibre bundles ,ith fibre F. Let Bun[P]X
be the subcategory obtained by hodi.ng X constant in d

and .ul~l b the subcategory obtained by holding B con-

stant in dc Let Pi be the "class" of all Pan[rpJ a:d let

P2 be the "clasr" of all un[F1 g. Then Ean[F] "= Pf X ,.

Proof. As in 1.18, it suffices to prove (3) of 1.4.
Suppose (uF,f): (X F/CG-pp, ,)---(X F/G,p'F, B'). Then we

have a unique Pf- factLtorization

(Xp, pp, 3)- -----~" __-_ (X;, 3pB')
(,f ) (up, I)

(X7p, ou,,B')
as soon as wee pove t-ha (XplouF,B") is a fibre bundle.

However, this is a simple consequence of the fact that we

are working rith quotient n>,i .e d the details are left to

the reader.

(1.20). Path Ct -. C Recall that a graph is a

class of objects, a class of morphisLs, a domain function

and C. codonain function; i.e., a graph is a category without

composition. Thus, there is an obvious forgetful functor

from the c.--'- y of all categories to the category of all

graphs. ( e should point out that a morphis. of graphs

consists of a pair of functions (F' F2), so that for graphs

G1 and G2s F1:Ob',-- bC- 2 and F2:Kor G1---:or G2 so that

if g is a Gj-morphisn, then fl(dom g) = dom F2(g) and F(cod g)

= cod F2(g).) This forgetful functor has a left adjoint;

i.e., free categories (over graphs) exist. (These are

also called oth catofories. ) This construction is done

by using finite sequences of "composable" arro-wrs in the

graph as morphl ns in the free category and juxtaposition

as composlltio:, Given an object A in the graph, ( )A (the

enpty sequence from A to A) is the identity morphism on

A. The path category on a graph G is denoted Pa C. Two

systems of notation -:ill 'e used in path categories. We

may denote morphisss pictorially as A-fi->,-2-> C--r*..-. ->D
or as sequences as (g;,..'~* 3 2:g ). Note that -hen writ-

ten as a sequence, the morphisms are written in their
Vcomposable?' order. The identity morphism on A may be de-

noted as A-)L or just by ( )A. We will riite A-l-B (

B--E0 as (g,f).

(1.21). Cateyorical Relations. Let C be a category

and let E be a relation on MIor C, Then E is called a

cate-o;ical rlation if and only if f E g implies dom f =

don g and cod f = cod g. Also, E is said to be co-ntiable

if and only if (1) f E gs hog defined, and hof defined implies

her E hog and. (2) fE g fch defined, and g'h defined implies

foh E goh.

It is clear that U(hoir,(A,B) x hoi(BA)) is a categor-
AB e ObC
ical, compatible equivalence relation. Since the inter-

section of any family of categorical,compatible equivalence

relations is a categorical, compatible equivalence relation,

any subset of U (hon(A,3) x hom(BA)) generates a categor-

ical, co ? LatL-le equivalence relation, Some sources refer

to a ca te-orical, compatible equivalence relation as a

congruence relation.,

Finally, if 2 is a categorical, compatible equivalence

relation, thc-- ::e nmay form the quotient category of C by E,

denoted by C/0, as foll s: Ob C/3 Ob C and Mor(C/J) =


(NIor C)/E, dao2aLrU- domr f an l coi? ~ f] od f, aa [fjo[ge]
= fog, whore 7 :'Trite w zirIn iYi E 9 -ith repJreoent alt ve
f in .a-s C ff).

CHAP7-_, .II


In this chapter we show; the equivalence of the general

concept of w;ell-factored category to two special kinds of

well-factored categories.

(2.1). Definition. A rultipl.ca tive ca~teory without

unit is a triple N = (i,0,<) such that-,

(a) M is a category,

(b) Q:i. x 1---4 s a bi functor.
(c) For all objects L,I,NI in K, rLNN:Lc ,.)---(LC )~ i

is an isomorphism and is natural in LI, and N. We will

usually vrite L,. ,N as just c<.

(d) For all objects L,i,N, and P of ~, the fojlorins

diagram coirutes:

L( 0( N ( )) ('0 (L(I)N)PP

L ((I)0? )--------- >(LO(I;N) )

(2.2). Re .r. Definition 2.1 and the definitions of

a semigroup and of an act are motivated by si._ilar defi-

nitions in 5] Conditions abh; a^n3. c do not imply con--

dition d. For example, let N i ,I. where R is a coinutative

ring viith id-ntity Let 0 b3 the usual tensor product

of raodules over R. Let < be defined by: 1(m@n)O---(-10ln)n.

Then conditions a, b, a-nd c hold but cd fails, if char R 2 2.

(2.3). DPfin.tion. A sei'!:r7oup in N is a pair (Su)
such that t-he follo.;ing holld:

(a) S is an object in 11 where =- (e,,NT ).

(b) SxS-~-S is a morphism in L,

(c) The following diasgram commutes:

S( S.3).----------....--- so

(S 3 )S

ul1 u

S@ S3--- S
Whenever: possible, re denote (S,u) as S.

(2.4). ^ A.-it.o. If N is a maltiplicativo category

without unit, then Sgp(N) is the category of scmigroups

in 1. A or-p' ...:' i in Sgp(N) from (S,u) to (T,v) js a mior-

phiswm f:S->T in so that the following diara c. ;-es:


TOS'------------- T
f~f f

T0I1 -11-------------> Q1
Composltlon is ind3ued from I.. It is clear that Sgp(-)

is indeed a category.

(2.5). Definition._ Let (S,u) be a semiigroup in.N =
(M,<,<) and let M be an object in I,. Then a left c.t.ion

of (S,u) on K in N is a triple ((Su).:s, -l---) :.hre z

is a morphisi in a!- i the- ll...i diagra coit

S (S ) .: -------------s- com

(03)0:: z
Si ______________________
o Cy. i .
TWhenever possible, r> :;ill denote ((Su),i,SM--M) by

(S,MI, z).
(2.6). DeLiition. Lact(N) is the category of all

left actions in N. A morphism in Lact(N) taking (S,M,z)

into (T,N,z') is a pair (f,g) where f:S--T is a morphism

in Sgp(N) a 1nd g .:--> is a morphism in M (JT = (,s-,
the following di-araa comimutes

s0:,T i 1;

f03 g


The com~ ~sition is jnducod from I1. It is clear that Lact(i)

is inlcee a cate-or y.

(2.7). .iflitio-. ei't '1 = (L,0,s) be a multiplicative
category .:itho-ut. unit. is said to be e::ct if and only

if c< is al:zays a identity morphism.

(2.8). P in o t.,ion. Let C be a category. Then there
exists an e:.;t ultipll.cative category without unit,

such that C is ioo .tc to a subcatc cry of lect(0,).

ionf01 ah :- t" the 3 c-~t ory of all e.njofunctors of C.

Then N = (2,composition, ~euality), is an exact multiplicative

category ith ct unit since t in-:techane la,.r says compo-

sition is a bifunctor takin x ~xM into E. Let C be an

object in C. T'-.: i et C.C-->C be defined by ~ (A) = C,

for each object A in C, and C(:) -- IC for each morphism

f in C. The;] CU is an endofunctor of C.

Let K be the identity functor on C. Then it is easy

to see that (K.1 ) is a semigroup in N. Define F:C-->Lact())

by F(C) = ((KiKI),C; 1-g). The follo-uing diagram coammutes:

1 i
1 01, 1-
Kv I
Ko C-- c----------9 .

This sho;,s that F(C) is an object in Lact(N). If f is a

morphirn in C, then F(f) = (1 ,) here = t'AA b and,

for each object A in C, tA = f. lt is clear that f is

a morphism in 'N and, that (1,7) is a morphisn in Lact(N).

Thus, F:C--l>ct(^) is clearly a functor and is one--to-one

on objects and on norz-' ,

Now ,e sho:- that ia(.') is a subcategory of Lact(N).

Suppose F(f)o?(g) is defined. Then cod F(g) = (K.,,1 )

and dom F(f) = (K,,1-) and cod F(S) = don F(f), Thus,

S= C so B = C. ut then cod g = B = C = do f so fog

is defined. Hence, F(f)oF(g) = F(fog) so ira(F) is closed
under composition. Therefore, ir(F) is a subcategory of

Lact(`) so C is iJ o ;orphic to ii(P).

(2.9). q RA .i. Although proposition 2.8 is a very

general embedding theoren, it seems to have very little

practical use. We wil prove later an embedding theoren

of uell-factored categories into categories of left actions

so that the enb.ddin preser-vs F'-factorizations. This

is a nuch more useful enscr din, Hoever, we will first

prove an embedding theore of a somewhat different nature.

(2.10). Tierorem. Iet C be a well-factored category.

Then there exist categories C C; and C such that C
e- there e1 -t -
is isomorphic to a subcategory of C x C x C which is well
1 -2 -3
factored by the standard :ell-factorization. Moreover,

each iP.-orphism in C maps to a P.-morphism in the subcate-

gory of C' x Cx C for i = 1,2.

(2.11). PyRr'r. Before proving 2,10, :,e apply it to

prove 1,5 and 1,6. Then several preliminary propositions

leadinc- to the poof of 2.10 rill be given.

(2.12). Helri, ote that if A is a subcategory of

C x C x C, so that A is :ell-factored in the standard way

by P, and F, then letting P = P2 and P = [, we have

that A = F rx P provided (3) of 1.4 holds. How-ver, P

and 2 are not the standard -well-factorlzation. It is

clcar that by letting CJ = C~ C' = C and C = C that
.a . z-2 ---3 -3
A is then r:ll-factored in the standard way by "1 and P

as a .u.boates-e_,y of" .f.C ~ By 2,10, given aany cate-

gory A wit h any ,ell-fac.torization, we can find an isomor-.
phic copy of A so t'hat the isomorphic copy is then well-

factord i. th stad.ird ni c- by the corresponding :1ll-

facto: a on,

(2.13). P~oof f 1.5 Definc H :S-->P >x by H(s) =

(s1is2), 'hee s = s2s1. (He are usin, subscript to
indicate .. -zlorphisri ) By 2.10, there cx-ist categories

C~ .2, a nd C. such "tht we have an eLbdd ing F:S---->
C1x C2xc^ th ch preserves Fr-morphisss. Thus, F(s2) is
a r2morphis s r. F(s1) is a r1-mIorphi~s i.e., F(s2)
(a,1) and P(si) = (l,b), for some a and b. The we have
F(s2)oF(s1) = (a,l)o(l,b) (a,b) = (1,b)o(a,l) = F(sj)o(s2).
Dat F is one-to--one o0 norphisms so s2o1 sl 2. We
know, H is w.ell-defined since the -P2 f actorization of
a norphisn is unique. Let s and s' be in S. T:. sos =
(s2o0s1)(s2os.) = s o (s s')os' = s o(sos )os' =
2 2 1 2 1 2 2 1 1
(s2s00o s,! Thus, H(sos') = (s1si s2os') =
(si,s2)o(s),s) = H(s)o H(s'), so is a functor and a
monoid +orphisH. H(s) = H(s') implies s = s20s = so
s = s'. Thus, H is one-to-cne.
Let (s1s2) be ai elenmnt of Jx P. Then sO is
in S and H(s2os1) = (s1,s2). T',-;, H is onto. Since F[ and
P2 are subcategories of S, 1 arind are subn-onoids of S.
Hence, H i.s an isomorphism and so S is iso hic to F x U.
(2.14). o- of of 1.6. e w:ill first prove that P2 is
a normal subgroup of G. Since P2 is a no'--_ty subcategory
of G, 1 is in r2 and f2 is closed under multiplication.
Let g2 be in f2, Then 2 = h2hl, whe:r h2 and h are

unique elements of P2 and respective y. Theni = g,2o1

i20h2oh) = ( 2h2)oh11 = iol 7 P- uni quen of rP 2
factoiza ion, i 1 so g = h nd h i in -
aco h = 1. so g2- = h2 and h is i

F2 is a subgroup of G. Let g be in V and 2 be in P.

Then by an argurlnt similar to the one in 2.13, w e have

that glog2 2 o2 Thus, for each h in F g, gohog1 =
o ho g gich I s in 1
2oglOhOg og 1 = 21 lP g2 2hog 1whiccl is in
f2. Thus, i2 is a normal subgroup of G. Similarly, Il
2 1-Y 1
is a normal subgroup of G. Thus, by uniqueness of 12-F

factorization, G = rx where x now denotes direct prod-


(2.15). DfJinition. Let C be a category wThich is well-
factored by l ancd i r Then a new category C. may be formed
as follows. Form a graph denoted as G1 by taking Ob G1 .

For each P -morphism g:A---3, there exist unique A and B

in P such that A is an object in A and B is an object in
B. Let there correspond a unique arrow g:A--) in the

graph G1. Define o on Pa(G1) by:

(1) (A A >A) y A A( ) A) for each object A in A.

(2) (W-- - ->U) y (-g- -?U) whenever gof is

(3) (--- ) () povided there exist Fi-
morphisms a and b so that the follo:Ting diagram comu:Ttes:


a b

W----->----------> .

Now u is a cat-sorical relation on Pa(G), Let v be the

catcgor.' cnly cop 'tible equivalent e relt: io:'n ;:.nated by r.

Then let Cj~ (Pa GK)/y.

(2.16). PF-.?-. TLt C be a category which is well-

factored by ai and 2" :r, e may form a category C2 by inter-

changin 1 and 2 throughout definition 2.15.

(2.17). o;oqiitop. Let Y be a categorical relation.
Let v be the categorical, compatible equivalence relation

generated by y. ihen consists precisely of those pairs

(gg') of morphisms such that don g = dom g' and cod g =
cod g' and the following holds: there exist morphisms

g19 o. such that for each i = l,...,n, dom gi = dom g
and cod gi = cod g and, for each i = 0,...,n, (letting

S= g and gr+1 = g') there exist g and' such that

ii+1 ii+1
gi = o ***ogi- and gi+1 = i oLi1 "og1ii andc, for
each j 1= 1,...,ki, E gg where E is =, or 1

Proof. The proof of this proposition is routine and

will be left to the reader.

(2.18). Fropostito;. Let h and h' be ri--norphisms

in C where C = r x y. Lot Pa(G2) be the free category
formed in 2.16 and let h s.nd hC denote the morlphisms in

Pa(G2) correspon:.ing respectively to h and h'. If hh',

thenl in C, h and h' are F-roroi.. '." grid connected by

P2-morphi. sms
Eroof. In Corder to prove this proposition, it is nec-
essarOy to m2. a careful analysis of what h ? h' means.

Namely, we he have morphisms gi'" as in 2.17. Note that

we may avoid the possibility that sone Ci is the cmpty

morphism by, if so, pu'iting -an ienti.ty (corresponding

to a P1-identlty) alo-r i"ae .gl' g n nsd a y-related path

categor-y idezntity alongside h and. h'. Nowu e write dowun
equivalent cc itions in C to describe the process in 2.17.
We have that there e-:ist seuences of P-orphiss SO,
Sn+ such that S = (h)1 9n+1 = (h). no Si is empty and

(since *einay i thout loss of generality assu-. that gi -
i-for each j except sonm one value of j) for S. and
Si+1 w;e have (or with S. and Si+1 interchanged) one of
the follo-.ing occurs:

(a) 1A ( )A r-eans that A is an object in A so -e
have Si (Vp ... ,k+v,vk+k,7 l. v) and

Si+1 = (p * *;k+.-s'LkVk, ..*v1) here ^k
vk has been deleted from the sequence, vk is an identity,
and cod vk_1, dora k+, and co = do! vk. are all in the
same P2-categcry.

(b) Si = (Vp,*.. sk+1 Vk"k-i, ..1) and

;-1 (vp . .,Vk .,stvk- ,. 1 ) where sot = vk.
(c) Si = (vp9 ...,Vk+.vv . vi ) lnd

Siv-"1 **. ,t1' 1.-1 . .v ,) ;:h e thore exist
[2-norphisms oa iad b so that asovk = o.b,
T: (a) corresponds to (1) in the definition onf,
(b) correspo-ls to (2) in the definition of (t and (c)
corres onds to (3) in the definition of Ty. ,Thus, by 1.3,
h and h' are P--nor-phiss grid connected by P2-.orThias.

(2oin9).. _-:'oCo to hon. Let h and 0h be --.. 1 "p-h-sns
in C T.here C x L et Pa( Gi) be the path category
fo'.-- in 2.15 and let -h E- h' denote the no:; -* :. In
Pa(G1) corres'-i: r ti to h n h, If h

then in C, h ani h' are P-sorpl'isms grid conne-cted by

Proof. The proof is th:e S aeo as for 2.18, :ilth 1 and 2

(2.20). Pr;f' c-" 2.10. Lt an be the cato
formed in 2.15 snd 2o16 respc _tivevly. Let C be the dis-

crete category for.7ed by the objects of C. Let A be an
object in C. Then there exist unique A. in [. and unique

A2 in r2 such that A is ar objot-. in Ai a.nJ A is an object

in A,. Define F:-C1-. x C2x C on objects by F(A) =

(A1 A). Suppose f:A---B is a norophisn in C. Thon there
xist a uique facto-'iza.tion f = hog -.he-r g is a r r--orphism

and h is a P2 -orphion. Then define F(f) = (Ch7[gC3):
(1A2A))----(B B 2,B). It is clear that F is .ell-defineSd

Now ie must show that F is a functor. Since A1 1 1l
A A 6
and since 1A is both a 1-or:phi2s an' a [2-orph; s, P (1)

(CL. ) (A., A) F (A) Suppos A-- -
Then we have r1c 2 factorizations s indicated by the fol-
loting co~uta-tive dia --r:

f f
6' h'


By definition of v in 2 15 and in 2.16, h=p' in Pa(Gi) and

pyFg in Pa(C). : [ h = [p ] l P: [J.

(f)o(f) = ([h' ,[Cs'])o(h],rs) = ([hoh][o, go]) =
([hrl [p [p]o[]) -= ( [h' p'C pog]) F(fo f). Conse-
quenbly, F is a functor.
It is cl-sar that F is onc-to-one on objects. Suppose
F() (f) where f and f' are moi-phisms in C. Then dom f
don f annd ccd f =- cod f. Also, f and f0 have fac-
torizations as indicated by the following diagram where the
top and bottom triangles are cormmutative:

r9 h
_g 2_2

Thus, F(f) (ChJ,r]) aind F(f') ([h'],7 gl]) so [h1] =[htl
and [zj = e ; i.e., as Pa(GI) ond Pa(G2) morphismis, hh'
and gCg'. By 2.18 and 2.19, h and h are F2-.:orphisms
grid connect by r-morphisms and g and g' are F--morphisrms
grid connected by -orp.his ras By definition of a well-
factored category, wie then have f = hog = h'oge =: f.
Thus, F is one-to-one on morphisms,
We next show that in(F) i.s a suboategoory of Cx Cx C.
It suffices to sho:e J.m(F) is dco under comooosition.
Suppose F(f)oF(g) is defied. Then doi F(f) (A1 A2,A)
and cod F(s) = i'B,2, B) so A = B, But by definition of
F, A = do:s f as, B = cod g so fog is defined. Hence,
F(f)o'F(c) = -(u-) so ij(F) is cloCed- u nicr counos.tion.
Since r: ha that C is i sor.c.chi to ir(F). :,; nowr

show; im(F) is well-factored in the standard manner. It
suffices to sho-: that each P.--noro'hism in C nans to a i-

morph is in 9C x C.x C, i = 1,2. If f is a r;-morphisn,
then f = lof is the fP- factorization of f so F(f) =

([f),[13) anid [1 is al -.-s an identity in Ci Hence,
F(f) is a r -morphi:-, Similarly, we can sho; that F pre-
serves P2-norphisms. This completes the pr-oof.

(2.21). Re l~ak. Now-T that we have gone through the
construction of C' C2 and C3 we may ask what Ci, C.,
1.- -3 L-1 2
and C are for some familiar cateSories.
Suppose S is a monoid wi-ll-factored by and r
1 2
Then as noted. earlier, discounting empty subcategories,

card( 1) = card( ) = 1 and, identifying 1 with its member

2 2
and P? with its Ynaembeis S x r as a monoid. Put by

2.10, S is isomorphic to a subcategory of Cx C2x C .
j.1 -2 3
We ask, "Ihat :are ~., C, and C'?" According to the con-

struction, C = (Pa G.)/Y whire GC is the graph of i -mor-
phisms. Then G1 is P2 considered as a gropho Bt we

identify Ip2 i7ith ( )GI and (f,g) with (fog), Finally,
we say ftg if theeare e -morphi isIs a and b so that bof =

goa. But as noted in 2.13, goa = aog, so bof aog. We
then have two 7 -F fractorizations of an S-morphism. Hence,

C1 2' Similarlly, C. 0 and card(C) = card(Ob S) = 1.
It is easy to see that F:S--C x C x C from 2.10 is onto.

Similarly, if G is a group and is ;well.-factored by
1 and P then card(P) card( ) 1 and, making the

identificti of above C., C 0 ,, -.0 crd(C3) 1
-. c 2 -. 3

No;.w 'we consider the category of all modules over all
rings. Ob(G1) ,jM' I is a ring- Arrows in G1 are _R
-- gso that (5,l):(Ri:, )-- :(ER,;) for some M.

Suppose (g-1)_(RI ,)--,-''E75 I'),. The following diagram
(, ,.)---- --- ---------( ', -)

(1,0) (g,) (1,0)

(Rs1, v )-1 ->(R Mi', .: )
Consequently, (G,iT)(S'(gj ). Given a ring morphlism g:
R-->R'- (g,i):(ER R.)--,(RHS~^,f,) where r-r' = g(r)-:ra

(6,1) (g',1)
Also, given ------- we have (g,1):
(, (.)-->(R ,i.S) and (gr ): (R 9 1,.7')-----R ?, ,9 ).
The following die.g-,. comrnutes ;-here r. r'm = g(r)Vsnm'

(1,0) (1,0)

'(R g)--- 1,-:R' ) .so -
Thus, ((g',i' ), (g ~l) )F(g'og, 1 ) so C, R.
G2 has as objects all RK such thab Ii is an abelian
group. Ar2 n G2 are RM--- : so that for some
R, (RM,)---- (RI,',). Then- the following diagra
(00M N, )---------- (, .

(0,1) (0,1)

(. ^, )---_--- c-_,>(,:i: 1 ..:.)
iiv Go(1p Ig)phisu
Conseuetly, (1R<) (1~0,c) G.ivn &niy group mc:;hisu

g:---:-I', where and '1n are abalian groups, ( (,g):(0,Mo)
--) *()) so (1),o .: -u'. Given ) v g

we note that ((1R: s'), ,))(1E Thus, we have
couposability of morphions. Consequently, C = (Pa G2)/P
is isomomorophic to Ab. Our:- results then are that for RM,
C1 C2 Ab, C is discrete, and card(C3) = card(R ).
(2.22) Propos..trion, Let N = (I,,,<) be a multiplicative
category without unit. Let (S,u) be an object in Sgp(N),
A be an object in ,1, ((T,v),B,z) be an object in Lact(N).
Suppose ft,(S,u)-->(Tsv) is a morphism in Sgp(N) and that
g:A-->3 is a uorphism in I. Then ((S,u),B,zo(fOl)) is
an object in Lact(Hi).
Proof. This is equivalent to showing that the following
diagra n co:-iutes:

10(f0) 10z
so(S)3) >S(>(T( ) >SO3
C< fl

.u01 ;u3z
S03-- T3B T>3 .
ful z

The proof that the above dlagrrai7 commutes is routine and
is left to the reader. 1o use that f is a semigroup mor-
phism, o is natural,& is a bifunctor, i.nd (T,Bz) is an
object In Lrct(N).
(2.23). oiqo-osijtion. LGo t N be as in 2.22. Let C =
Sgp(N) and C2 i Let C, be the discrete category with

objects z:SOSl---A so that z is a rorph'.I.n in 1. Thcn Lct (l.)
is .:ll-f].ot. d. in the standard Iannor as a subcanegory

of x C x C2 ':3. 2o will refer to this as the standard well-
factorization of Lact(_).
F-roof. By 1.8, it suffices to sho.; that (3) of 1.4
holds and that if (1,1):(SA,z)--->(S,Az') is a morphism
in Iact(), thein z =Z'. Given the morphism (1,1), the
following diagram. co:imutes:

S--A ---->A

S 0A-:%A .
101 1

Thus, z = z'. Suppose (f,s):((S,u),AzA)-->((T.v),BzB)
is a morphisn in LBct(N). By 2.22, ((S,u),B,zBo(fi)) is
an object in Lact(N). Aso, gozA = zBo(f), since (f,g)
is a morphism in Lct(I). Hence, gozA = ZB (fi1) (1i3)
so (.i:):(S,~ A)----:S BB;zB (f0i)) is a morphism in Lact ().

It is cl].er that (f,1):(S,,zBO(z fi))----(TB,zB) is a mor-
phisn in Lnct(!_). Also, (1,g) is a P---orphism end (f;1)
is a P-. orphls" and (fsg) = (f,1)o(1~, ). To see that the
factoriztion is unique, it suffices to prova (S,B,zgB(f@l))
is unique. If (f"g) = (f',1)o(1,lg), then f' = f, g' -- ,
and dom(fC1) = (S,Bp). Also, lop = z B(fol) so p
zBO (f01). Thus: the i-P2 factorization is unique so Lact(fN)
is wel). .l-factored in the standard mnsinzr as a subcategory of

C x C x C.3
(2.2!43). "Th \r. Let C be a :ll-factored in the stand-
ard rcJ.mner subost :- of C x Cx C Then there exists

an exact u.1tip'l te ,-c T -~ (;tc<) Tthout' unit

such that C is isoorphiLc to a ,ell-factored subsoatcory
of iact(JS ) so that the .'ill-factorization of Lact (N) induces

the el-fcton of the subcategory. Also, each f-
lorphism of C maps to a s-:aorphisnm n Lact(N), i = 192.

(2.25). e--' EBsfore proving 2.24;, we obtain a pre-
liminary result. In fac'i this no-r summarizes 2.1.0 and 2.24.

(2.26). eot. Lot C be. a category and let PF and

2, be 'classes" of subcategories of C. Then the following
are equivalent:

(1) C is ,sell-factored by F and r.
(2) Thero exist categories C C, an. C3 such that
C is isonorohic to a subcategory of C, x C2 C3 rhich is
vwel l-factored in the standard manner and such that each

r -norphi~si in C naps to a P.-noropis n x -2x 3.

(3) T 'e exists an exact nu.ltipJlicative category
S= (i,3,c<) .without unit such that C is isomo:p:hic to a
suboategor.y of I.Ct(!) ;ell-faotored by the factorization
induced b; the nl_-factorioation of Lact(N) and such that
each r._0.rphis in C 1i ps to a a -orphisR in Lact(i),

for i = 1,2.

Proof~ It follows from 2.10 that (1) implies (2).
It follow s from 2.24 that (2) implies (3). It is clear

that (3) inplics (1).

(2.27)o. Y'cof of 2.24. Disjointify the catoories
C. and C2. Form a gri:',l as follows: objects in P consist
of pairs (A1.A2) where Ai is an object in C, for i = 1,2,

or objects in P r T: th -selv.s bo objects in C.,. For each
object (A~,A2jA-) in C, Ie let there cor--espcnd a unique
1 2 -

morphism front (A1,A2) to A2 whichh we will denote by A,
Now we form another graph G as follc;3: Objects in
G are n-tuples (UlJ,...,U ) .There each Ui is an object in

01 or C2 and n is greater- than or equal ons. A morphism
f from (U1,...,Un) to (V1' ..,Vm) in G is a set of morphisms

(in Ci, C2, orP) denoted fi or fi +, here fi:Ui--Vj(i)
is a morphism in C1 or in C2 lnd fi, s (U. )---T (i)

is a norphism in i. Also, we require n>m and that, for

each i = l,.. r n, i appears c:actly co anco a subscript

of sonm fi or fi i+1 .There fi+1 is covtntedr as having

two subscripts.

Given an object (X!,,..,X) in Pa(G), denote Q :

reU tp-.e f and g
(X1. .,qX )-->{ ...,X ) as 1( ')e. Suppose f and g
are two morphisms in Pa(G) going front (U ,..Un) to

(V1,...V ) We obtain from f a ne:; n i! i. ,F
and from g a new morphis:u G = (Gs,,., G1). Suppose f

(U, Un ) Then 3.et F = ( (U1 .. U)). Otherwise,

f = (fk f1) Insert bfore f1 the norphisna
S(Ul ,, ..Un)
and after fk the morph-is 1 .)o For i =I ,.,.ck-1,

let cod f (Uiu ... -Ui)). Insert bset'ecn fi and fi'+

the mlorphism 1(U .U Cslling this ncw sequence

(Ft ,..,P1) -e see it is a path in Pa(G) and we let F =

(Ft~ ~oeF1)) G is obtained from g in a similar manner.

Let iO be such that liCr< L. :IU F> ----> ; in fact~
', i0 "O
F 1i Lt I-, 0 Exactly one of the following
J0 = i 0 L0

possibilities occv.rs
2 _
F i:U ---.- : (UiU )--"U I or, finally,
0 0O o0

l0-1,i 0 i(U 01 )--1 p Whichever of these occurs, rwe
call it F~U- and P = Let F F
0 P P P p p p o
There are three possibilities for W which are analogous
to the three po.si. iti for U1 hichever of these
occurs rwe call F.'. Continuing in this : w ;e obtain
the sequence (t,,..,F 1). For e = 1,,..,t-1, either
0e 0 +o e O ot er
F0 e 0 is (defined in C. or in C or one of F~' and F0e+
0 0 0 0
is a r-morphism. By compo3ing ;hatevor adjacent norphisms
are col: ,sable, we obtain a now sequence (F. i0, 71)
-0 0
so that for l6cT -., exactly one of F? and Fe+1 is
0 "-0 0
orphisr. Similarly, from G we obtain (G 0 o,,G ).
O 0
If, for 0 = ,n, (FTiO (G0 ...G ),
then L-e say fyg. It is easy and is left to the readr
to see that Y is a categorical, compatible equivalence

relation. L~t A = (Pa G)/y.

Lot h !/j where fp is the categorcal. comatible
equivalence- relation generated byp .: catcocal la-
tionp consists of all pairs (aa') such that aa
(U,"c..,U)-r-,.(Vi',* ) and there exist ..- native

b and bh in Lor Pa G so that a = b sid a' = b' and
either a = a' or at least one of thce follo-ini t-- o condi-
tions holds or with a and a' interchan ~d at least one of

the following toco conditions holds:

(1) b = uobbodt, blb2 C::or G, ffrbj f1:X--- ,
f2"i"'* fb2 Cf (;bj C4 11 "---,v b0
ub!bod b:b;oar G, foEb q (X.-g- f ^2
2f -t .2 -0 J o tha G (= b 1 1 1
2 ,- ik 2

(2) b uob2obldc, b bE..:or- G5 fIl,f2 i3b1 f1:Xi >Tj5

f2 'X-- j t fl 1' 3 ara irdntltics, f4c-b2
J f3 1.++' 12f 329
and f4:( )-->Y b' = uT 'o bb- ', ,fbI, f

X --, is an identity, f(X, ) and fb

so that f:('! T )-- Also, (X 1,X

-1 X ) n Ykj. = Y
.... "k = 'b

Pictorial.ly, co-litions

(1) and (2) are as follo-s:


(U1.66 ee U1)


See I

(Y, .....0

(V 9......
(V ..

+1, Co. ,x)



.... .*,V )

here (XiX~ ) (X


( U10

Sd o

( I . -c.6

(VY . . . .


0 0.e.


(U .. to . .... o o Un)
(x{ c 0 x a )


1 5 9 T. 9' 0 0 a I ,


(Y 1 ... 'Q C Y- ,)


(V1 0.. .c ........cV0)

qUn) (U1) ................. ,,.Un)
Xx) (Xi' X X- 1 __-X.. +1X7 .

1 I2
-(-w c T. F a-.

,") (Vw..i ,", .)

VI C 9 o c c o ( Vii
-i V;

where fcf:-".. and "i are identioc, (. ..,X, ) =
^ <1 1 .,- A J. Y

i+ + 'i Ll .

Let (UI"..,U ) and (Vl',.." V ) be objects in G.
Let (U1,i... .U ,1) = (U ,.. L,UV),, V ); ie.,
let it be ( 0...,~,ln+m) rhere, for i = l,...,n, W J = Ui and,
for i = n+1,...,--, Vi Similarly, if f:(U1,...,U)
->(V1 I. f:(U ..,U)-0 U(V ...,V ,), f =f U

f1,1+- an~ f = f ] then f f :(U ,...,Un)9
(U ... ,U )--(1,.., V)(V ,...,V s) w-here ,e let f0fC =

where, for = 1,..,n, g = f and g .i+l and,
Or i fgi
for i = n+1,...,n, g = f and g = f .
Nou :e e fie @:xi---2.: On objects, 0 is the sa-S

as for G. Given [[Cfl and C[]s th3 n f -- (fk 0f
and g = (gP,..,, -i). Assume that kp. By definition of y ,
we see that if (X1..,. : ) is cod g, then g'(,...,l,gP,...,gl
where we ta: e k-p 1(XI ,s a,d wher (X i...oX ) -
cod GP = cod. g, He define [fJ.13@[[Cs] = [[hC] where h =
(f 01.,- C, 0'1 z- '-..f 1). If p>,k, then us lengthen
f similarly.
For noto tional conyveience, ;e vill 5-ite a (1) b to
indicate that a and b satii y condition (1) in the defi-
nition of Ue use a sil :.'l- n ot.ation for co-dition (2).
We need to sh- r Q is all-defined on corphisrs. S 1)ose
[Cv33 = [[v] c '* [[u]J =[Cu'3 We r.ee:. to sho:r [ Cv3]0 [[ul]
I= Cv][C[u'l. Thus; it suffice to showu CLv] [[u1]

ECCv lJ@[[u]] and [Cv'33CCuj = C[[-,, ([uC3 Since the
proofs are similar, we shoC 0 only thCt CCv]3[iCu]3 = CC[v]][[ul1

In A, we have C -v]-vT'3. Since is a c.t3 ;oricl; co-i pat-
ibleS reflexiv, sy. stric relati- [ v J]v'3 if fnd
I Sy2_, v -eJ;; j nd

only if there cxist VlI ..vn so that C[vfP7v)f[v2jo.. [7'1
By the definition of p, ECv]r[v means that there exist
representatives e and el so that [e] = [v] and [ell = [v1]
and condition (1) or (2) in the definition of p holds be-
tween e and el or that [v] =[vl]. How-ever, we may assume
without loss of generality that e = v and el = v1. However,
for [vl-]p[v2] since we have already used vi in conjunction
with v, we must use a new representative for vl say v1.
Thu.s, we have v = or (1) or (2) v1lV1 = or (1) or (2) v2
v2 *'"n = o2r (1) or (2) v'. Thus, it suffices to show:
b = or (1) or (2) r implies [[b]3][Eu]3 = CCw]]3 Lul1 andi
that b~w.: implies [[b]~3[[ulJ = CC3[w3 [[u]]. To prove this,
suppose b (1) or (2) U. Then b = (b k,.osb ) and w =
(,..u = () Assume k.,m and p>m.

Then [[b]]J@[[u3L = [[(bk 1.. b1 lbm um...eb u
and [[wlO[[u]] = p[[(wp 1 o,,-: 1,w u1, ,..l U 1 m.
EBt then (1) or (2) w-ill hold between the representatives
given above for [Cb3]0[[u]J and C[[w3][Lu] so [[b]]3[[uu3 =
C[[l[[u-]] 1;'-r! other possibilities for k and p with m
lead to differences. only in notation. Now suppose byrw
Then, recalling the definition of we see that composi-
tion of morphisns of the form 1 1. vith either b or w
preserves the relation y, so [[bJ] [[ul3 = [[C]][[u-13
Thus, 0 is Tell defined. It is easy to see that 0 is in
fact a, funictor,
Letting c< 'c equality, N = (MT,,c) is clearly an exact
mult 1prlitctive ca ;ocry l thcut unit. D"fine, F:C--->Lct ()
as follo ::. If (A 1A2,A.) is an object In C, let F(AuAsAq )

- ((A,(A1,A1) uC1 ,i ->A ~) ,A2, (A, 1,2 ) -3 3
Note that we now identify Ai wiith (Aij), i = 1,2. Also,
we identify f. with (f ) where fi is a Ci-morphism, =
1,2. We do this since this identification embeds Ci into
, i = 1,2. Given (frf2 ):(A1,A2,9A)----(A{,A ,A2) in C,
define F(f.lf2) = ([[f3 [[f2] ). That (Aj,(AA.) [[1,1]
is an object in Sgp(L) follows immediately from the defi-
nition of y That F(A1,A2,A3) is an object in Lact(N)
follows from condition (2) in the definition of, That
[[f]] is a morphism n Sgp(N) follows from the definition
of That ([[Cf l [[fJ2] ) is a morphism in Lact(N) follows
from condition (1) in the definition off. It is now clear
that F is a functor.
Suppose F(f)Fo(g) is clefined. Then don f = (A1A,A2 A)
and cod g = (A.A2,A 3) for some (A. AnA ) in C. Thus,
fog is defined so F(f)oF(g) = F(fog). Hence, the i.ase
of F is a subcatesory of "lct(o).
Suppose F(f) = F(s). Then f = (flif2) and g = (g1g2)
andnd [[f2f C[- 2] in i. Thus, [fj] f=C,
Thus, as noted whenn proving 0 to b3e well-defincd, there
exist hi~,1,h' 2h2,...hnh so that fl or (1) or (2) hi
S=or (1) or (2) h Zrh2 ".hn = or (1) or (2) gl.
Since doi fl has length one, (1) and (2) cannot' hold, so
we have f h = hi. As in the definition of y we obtain
1 1
from h1 and. the o lrph1..H) en (from 1)
the morphism IH (Hi, ..a ,)" Then since th~e .en-"th

of dom h1 = 1 and. dom = dor Hi. = don H. = dora 'i ^
1 1 .. 1


have H o.co = Ho ... 1. Since fl is a morp-hism in

C and fl = h!, than H1 = (i,h91) so h = HtO **
Cortinuin, in his nanrr, :we obtain f = h = H o*o
H 1 1 1 1
H .". .oi . = .2= g1. Thus, = g. Simi-
larly, f = g ence f = g. Thus, F is an embedding
since F is one-to-one on morphisms. It is clear that each

Pi-morphism in C maps to a Pi-morphism in Lact(N), =

1,2. This completes the proof of 2.24.



In this chapter, we will be concerned with a type of

varietal category introduced by Herrlich in [1] which gen-

eralizes the 'varietal" ca-L ories of La,.rvere and Linton.

(3.1). Rrik. We recall the follo-iing definitions.

(A,U) is called a cop-'-~' c-at ..--. if and only if U:A->ns

and U is faithful. If p,q, and f are i or.:i.-': ,3 in A A any

category, then (pq) is called the c-- -.1- i r-eation of

f if and only if (pq) is the pullback of f with itself.

If A is any category. f is said to be an E:^r .l..-.-.

a-: -' if a6n only if f is an epinor-phi"A: S- Il f = Log is

any factorization of f so that n is a ronororphism implies

n is an isonorphism. A norphism f is said to be a r.. 1- alr

e-r:\ if an1 only i f is the coeoualizer of some

two m orphisms. It is easy to se that if f is a regular

epinor~cr .... ', thce f is an: eztr;~ 1 epinor' c

(3.2). Dn fiJito (-errlih ). Let (A,U) be a concrete
category. (AU) is said to be an '- c t-. -- if and

only if it satisfies the follo:.in three con .itio

(Al) A has congruonce relations arrd oeo cr.izers,

(A2) U has a left r. joint.

(A3) U .-l. es-erves and ref3>lct-s "reu lr epi opiss.

(AsU) is said to be a vaijtais.t catepo-ry if and only if
it is ale~braic and satisfies the following condition:

(V) U reflects congruence relations.

(3.3). ES:v2le (HErrlich).

(1) The following 3ar varietal categories: Ens, cat-

egory of pointQe sets, category of all groups, category of

all semigroups, category of all monoids, R, for

any ring R, category of all unital R-nodules where R is

any ring with identity, category of all lattices, category

of all Boolean algebras, category of all compact; Hauslorff

spaces, category of all compact, Hausdoorff groups, and Abe.

(2) The category of all torsion free ab.elin groups

is an algebraic category uhich is not varietal.

(3.4)t emai'rk. We now prove a sequence of propositions

leading to our sufficient condition for a well-facto.-'

category to be algebraic or varietal. We state this suf-

ficient condition in 3,5. T first fe- propositions will

prove 3.63 *which is an important part of the proof of 3.5.

It may be helpful to tl: reader to verify the folloTingr

propositions only for the category of all modules over all

rings or for the category of ronoids acting on pointed


(3.5)* 1"-. '.2 Let N = (nI,,Qo) b:C a multiplicativo
category .nithoui unit such that. 5 has finite coproduc'ts.

Also_, assure that for eachI object I in I! that i;_. and 0:.!

preserve fiite coproduc.ts. Let U:Laot()x be

the obvious fo_ .tful functor. Ass;ie the follo-?.ng to

be true

(1) If f is a x-egulr cepinorphinm in Sgp(I) and g is
a regular epimorphizm in 1, then f5; is an epinorphisn in H.

(2) (Sgp(1!)sU2) and (W,U3) are algebraic categories
so that neither U2 nor U- assumes the e;pty set as a value.

(3) lact(2) has c ."-'.sce relations and coequalizers.
(4) If a and b are regular epimor;phisms respectively
in Sgp(W) and in [> and v is a nonomorphism and the dia-
gramL belo:7 consisting of the solid arror:s commutes, then
there exists ZA B:AB-->B so that the entire di--ra:'i in M
a0b u@v
A PC03c---------A> ---------------A "03 "--

zA',B: zAB zA' B"
b v
B B- -" o

Jpet U = T[o(U2xU )oU :Lact( N--->iis, ,rhere 7T 3Enxs--,EnsTT
is the product functor. Then (I...ct(),U) is an algebraic
category. If, in addition, Twe assume that (Sgp(N),U2

and (i U ) are varietal categories, then (Lact(:),U) is

a varieta! ca'.. ory
(3,6), PrcnosoJi.ton Iet N and U, be as in 3.5. Then

U1 has a left adjoint.
(3.7). Not~ ion, We wrill let Ni U1, U2, Uj and T be
as in 3.5 for the rest of this chapter. I t uE denote

an injection into a co-rroduct involving E. Define dA.B C

(A3B)l(AC)-->AO( OL!) to be the unique morphism d such
that douA03 = AuB- aeid dOuA@C = A*C. henIevcr possible,

we will denote dABC as d. Sim ilarly, we define cB, C, A

(EBA)JL(CCA)--4(BC)A, Whenvor possible, Ce denote C

as d. The assumption that AO_ and 2@A reserve finite
coproducts s..1': that d an d 6 are iso:norphisms
(3.8). Po-oit'.o. The transformaations d. and d are
1'rPoof. The proof of thi-s proposition follo-.s in a
straightforward manc'-r from th3 definition of a coproduct.

(3.9), TPo~.s o ._o. Tne following ito dia-rams commute:

A C( F^)$L(B D)3

A B C9B D 'dB, CD


d.A B,C sD
[(AOB)9 ] AA3)D]aA B (D3)0( CAD)


dA B,B CD, aB,CG 1

[(A B).] E Dc D ( (AiB) 0) J

I< Il c< ^

[A(C(D)] c-.)] aABC D --- >(AjIE)(C.)
P oo To see that diagram (1) co -os, it suffices
to show; that (1) comrautes when composed wiith the coproduct
injection into the coproduct of A&(5W,) and A( -).,
We 1ill prove it for uA.O- l ') only. We calculate the fol-
louj eq--ation, do ( J. )o" = do (u u '

C(161)0eUl C = (1C'1 )) = -L o -* ty

= 0 (10.d)od uA@(O3). Thus, by definition of coproduct,
(1) comi-utes. The proof that diagr~a (2) com-ubtes is anal-
(3.10). ?-.roc^Iticn The following diagram comlutesS

EA( BD)] 1J [A( C.-J)] --------- >[[(A 3)D] (A)C.
A,B DiC D {A B,A C,D
A [(BD )(COD) [(A)3 )(A3)] C D

I0dBtC,D dA BA C@1
AO [(BJ0C)0D- 0[AO( BJiC) D
PEoof. As in 3.9, it suffices to prove comnutativity
when composed -.ith the injections into the coproduct of
A(Pf.) and A(C' )). Since the procnedures are the same,
we will consider only uAO( )" Then we have the following
qualities: o Dco(i(eouu E )) = uc (0 u i)) = ((l0u,)l)<
iD0 iB B 'B
(d0l)(u 01)oc< (al)oUc(A)~* = (Cl)'o (I)u
Thus; the diagram conmz.uts.
(3.11) roposio~:i 0. Let R (RsP) be an object in
Sgp(N). Let I,- be an object in M. Define F( I,) to be
(R,K'II(~i::),z) where z is given by R(MI(?.^D:))-- d(RB;I)JL(R@(R-O))
-1 >(R:.i)lL( (RI) ( p01) : :(ROI ), i hore

UROI is the coprocduct injection. Then F(R:M) is an object
in Laot(i).
jroof- It suffices to sho tat t ht e following diagran (0)
co=ut.es. Diagrai (0) is reraw~ in ~dig r (Ot) a;:ith the
values of .z indicated, Also, in (0'), die". (0) is sub-

divided into cight subdia'3r- s,~ each of uhich is subsequently
sho-wn to co:'iube. This conc-",ibtucbes the proof that (0)
com utes,


Re[(MO.(RE:) )]

l 0z

O(H0 H(M ,'::) )- --->-M:,I (W;(._(Ai)
Subdiagrsm 1 in (0) is redrain in (16) and decomposed
into diagrams 1.1,1.2,1.3,1.4, and 1 5. Diagrams 1.1 and

1.5 comcute by 3.10, 1.2 conxutes since c is natural, and
1,3 and 1.4 co:iute by simple computations.
For subtdiagS-am (2) to co ': :, it suffices to sho;

[(l,p)lo = (lp0i). It suffices here to show equality
when we com- .-; on both sidos with the injections into the
coproduct of ROI and (ROR)ON. Since the p.ro.eclures are

similar, ow consider only UEp -. We have the follo-::ins:
[(1,p)&if'o .i = [(1,p)l]o(u 1) = C(l,p)aou l = i1

1 =(1,p@1l)ou. Thus, (2) commutes

Diegram. (3) co-r.u~;es by the definition of d and diagram
(4) col"luuli-'e since d is natural.

Diagram (5) is redra-,m as dia-ram~ (5-) and is decom-
posed into su" 2li--g s 5.15.o2, and 5.3. Diagram 5.1 coi-
mutes by con-lition (d) in def.in. on 2..1, Digrac 5.2
cori--utcs 5nri? c< is natural. Di c 1 5,3 C a ;ctCes since

(Rp) is an object inl S-p(Ni),


_. _-_-________- --- 1_ `'-__ ------ ---

o co
=1 'j a: :jti / i

T o "7M /
,LT~ v-: "

5 l.5 / /
II RI 1t

V/ -- ._i
./ '(.\ -.
/ / /C-. -
/ i

W g M5 91 ;

-. l5 -^^' ^ c- .5?
^ ^ , \f r RI Q) g

=={ f- = 5- I 0 5

J N.YL, -^lJ^^ g ^
RI I II (1@
~1 01

'H -' ,/-'~ ~ _
FlL 01 cr 1 0
cr1~~ RIR R r
5-b LJJ '- RI

/ r r" /i /iw
5--- C3 5--' I .r I

/ r C^ ^ "- r!/(

r RI, R^iI. / -
RI~~o ~ -: =

a i -! 'H~ -I l-: 0
j -- | _i _J j LI
RI S: C7

p"1 / cr! t


.9cc r-
rr^ 0 -

rr- 0 0

- I CO
rd d -1

te: pL

C11-*< ~- -
r-1 *-i == 0
zi32 -i=

rd rl

'- 121 >

0 0
p.; t\KCi r


\ \ -
\ \ o

clas ci

\ l\ rl

--S c

^~ \ \^^ :

v-IY L-.
\ 00
-5- -----> 121
\M arz v- L .

4 \j w, \
r- \- 0I \r





"a a cc c^
(-- < -i


01 04 01 \1

1 I g i -
/L r;
^-~V_ S ? &"i iI

We note that dia(r-n (6) corrnutes since .d is natural.
For diagram (7) to co-~ r.-auo need (1 p)Ol]oc = (1,p31).
By the definition of co-roduct, it suffices to prove the
above equation with u ,. or -.ith u -),. composed with
both sides. o .e obtain the equations: C(1,P)i ] ouR.

( (lp)l)o (u fl)) [(1,p)ou-0 = 101= 1 = (1,pol)oupO1.
The proof is just a.s easy for u( ) ., and is left for the

reader, Thus, diesram (7) co=mutes.

To prove that diagram (8) commutes, we adjoin uRv. 1
and u .o)01 to the diacrr''. to forn respectively diagrams
(8A) and (8'). It is clear by inspection that (8A) and

(8B) commute. Thus, diagrams (1) throuSh (8) commute so
our original diasram (0) conautes. This completes the

proof of 3.11.

(3.12). Pr (^osititlo- G iven that (1,g):(R,~ )--)U1 (ENz)
is a morphisr: in Sgp(N)xi, then there exists a unique Lior.-

phism (Ih) i- JL- (ct(Q) so that the followiL di. cgrcam coc utes:

(1 ,v. )

U (RNz) .

Furthrmoor:e, h (g z o( 10)).
Proof. Co ~rutativity of the above diagr-am is clear.
We u.st sfho (1, (g,zo(1'5)))) is a morph.is2: in Lact(N).

It suffices to shot: the ditc. --* below coriautes:
-d 1 ( '(1, p i)

R^__ -___- -____i^ iaz^




=0 "

0i 0 *

@,, 8 .
---I \ / 1

- C O-1
~01 ** 1

(X! \
\ Mrl r^ ---

V: 611

0 t -I-
'-I 0*
\n-i -

0 0 '-I '1
* -~ -* 0

n-icl 0 vi
'-I -- --- 0-
P1 */ PPl vi
Ir, e- x --^'-

`- -I .^---^ c: =

LJ %- 14- ^ .
/ ^----'^^^

/ ; ) C-.
/'*-'----*-----^Pl 1-

S /c.-
^ / ci

V I=

Ph Ph P4

Ph 04 oh ----- ---
L~J ., I-I

As usual, 1.7 provo cori'unativity by adjoining first u,,.,
and then R Wc obbtif thI equations: z -(plz o( ~g) V
We o U nthL [0g
o de -a z El [ 0 (qz 0(1cD))1 a(iC>,[Q) 0 um
zo = izo(1&c~ I = Z -(101)) o (l I p&l)-UROu 1o

(z l))u (Note that -,e sec
fro-- this ay?.: th-, :c ',iuad~ come mtaativity that h is unique.)
z 316,zo(1Oc ))]odou,, () = zFlo :7:zo(12:)0 1( 10&z) 0
C,= zU[(z (1Qz))j = Zo(1OZ)o[1(1(lOg3D

Bult. ( G)ROM) = (zo(1g))
a (IpBO-)-VB(n)T)O OC=zo(g1)o o = zp(p~-)oo. Thu s
it sufj ces to Drove tho diagram belouy Ith sub-diagarams
(a) and (b) conn-iuteo-

{ (a)->R IT


(b) z

Diagx~z (a) is cuxpmi&ed b~lol amr clearly commrites:

I p01

Diagrei- (b) ccl `;es sirce (R,i\,z) is an object in Lact(.
Sinc -,Tc- have ino`L-ed-u that h i'Un? it- is easY to see that

(l,h) is unique. This completes the proof of 3.12.
(3.13) P"roap--''it. Le (t (i(R) denote the standard
r.-subcategory of Lact(N) determined by the semigroup R.
Let Up = Ul ( Then Up: F (R)--> R&ix has a left adjoint

F. F(R~M) = (RMiU(Rc:,),z) is the same as in 3.11. F is
defined on morphisns by:
(R,) (--) ----------- R, I (RM) zy)

(1,g) F(1,g) =
N (1,(uNug,zN(10(uNo g))))
(R,N) I ->(R, N(RON) )
Proof. This proposition follows inr.ediately from 3.12.
(3.14). rE~ k. We no-ea pause to prove some propositions
of a different nature and which will be needed for the

proof of 3.5.

(3.15). .i'"* --'- ;o'--. Let C and C be categories wel1-
factorced respcti- ly by 1 2 and 1 P Let P be a class
which indexes both rand f. Let T:C--)C be such that
for each p in P, l:Gp-- p Let TG = T Assume that
K p P
for each p in P, there exists Sp:Gp---G so that S is left
adjoint to T Also, T(C) is an object in Gp implies C
is an object in GI, Finelly, if f:A--T(B), then there
exist unique f and f so that f is a P-morphism, f2
is a P.-nmorphism, and T(f2 )f = f. Then there exists
S:C~-)' so that S is left adjoint to T and, for each p
in P, SGp = Sp.
Pro ?f. Let A be <,n object in C, Then there exists
a unique Gp in P, so that A is an object in Gp. Define
Ip Dein

S(A) = Sp(A) Thus, TS(A) = T S (A), so there exists v:
A----TS(A), a ncrphi.u n Gp, so that if f:A-->T p(B) is in
Gp, then there exists a unique g:S(A)-- B in GP so that
T(g)ov = f.
Suppose f:A--T(B) is in C a9nd that B is an arbitrary
object of C'. By hypotLli s, there exists unique f,' a
G p orphiSm, and f2, a r2-orphism, so that the following
diagram commutes:
A-- T------(C)

f T(f2

Then T(C) is in G so C is in G1. Thus, T(C) = T (C) and
we have that there exists a unique g in GI so that T(g)ov = f"
Thus, the follo-ring cliagra cc utes:
A>TS (A)

Sfl f T(g)


f/ ( T(f2
T(B) .
We now prove that f2og is unique. Suppose T(h)ov = f.
Then there exist h1, aP orphi, nd h, La r-Lorphism,
so that h = h2oh1. Bat v is a Gp-morphism and so is T(hl),
so f = T(h2)o(T(hl)ov) implies h2 = f2 and T(hl)ov = fit
Then by uniqueness of g, hi = g so h = h2 hi = f2og.
Thus, by the front adjunction thoorem, S is the left adjoint
of T. Also, S is defined on norphi:.. via the front ad-

junction diE.agrm. Since Sp is the left adjoint of T ,
S l = SP, for each p in P.
(3.16). .gAviopr of S. The functor S is not necessarily
well behaved cn P2 ctegories. Tn fact, it tay be that
S(G), ;here G is any P2 subcategory, is not contained in
any P2 or .' subc.atcgory. ec no- give an example of this
a 2 i
situation. Let A hate objects A and B and non-identity
morphisms a:A---B and a-l:B-->A with the indicated compo-
sition. Let C = C' = AxA hav-e the standard well-factori-
zation, whert P> (C) deno--os the i subcategory of A/A deter-
mined. by C, an object in A, Let SA: P(A)---P (A) = p1(A)*
Define. SB': ()--- ^(B) as follo-7s: S(B(BA) (BB), SB(B,B)
= (B,A), SB(I(BA)) 1(BB), SB(i(B,B)) 1(BA)' SB(1,p) =

(l,;pI-), and S (1,p"l) = (1,p). To see that SB is a func-
tor, :o compute the folloA.ins equations. SB((1,p) (1,p-1))

SB(ii) = I(B~) = (1,~l)o(1p) = SB(l,p)oSB(19p-1).
Siilarl.-y, SB((1,pi)o (1,p)) = SB(lp--)oS B(Jp). Thus,
S is a functor.
Let T = IAxA .It is clear that all the hypotheses
in3.15 are satisfied, -iith the exception that SB is the
left adjoint of T 1 (B) How:ever, this is a consequence
of the front adjunction theorem and of some easy calculations
and the details uill be left to the reader. Thus, by 3.15
there exist S:C-~ ~0 so that S is a left adjoint of T
and S q(f ) S and S -(B) = SB. it S(B;A) = SB(BA)

(B,B). Thus, (B.A) is an object in P (A) but S(B,A) is not
an object in [!(A), _n foct, S(B-A) is an object in P (3).

Thus, S( 2(A)) is not contained in (A) or in P ().

(3.17). .?'oi ..il, p ..'re exists F :S3p(N_)x7--9Lact ()
such that F1 is a left adjoint of UI andl F ,I is the
same as F in 3.13.

Proof. By 3.15, it suffices to siho- that for (f,g):((R,M)
-->Ui(S,.Nz), there exist unique pl:(RMI)---3UI(C), p1 a

r1-morphisrm, and p2:C-->(S,N,z), p2 a -morphism, so that
(f,g) = UI( 2)op It suffices to show-. this since by
definition of Ui and by 3.13, the remainder of the hypoth-
ses in 3.15 are satisfied.
Let C = (R,Nzo(f01)). By 2.22, C is an object in
Lact(N), Lct pl = (1,g). Clearly p, is a [P-morphism

in Sgp(H)x.I Let p2 = (f1l). To see that p2 is an act
morphism, it suffices to note that the folloT.-ng diagram
RON f --------- -;> S:OzN---- A

f01 1

SN- .--.

Thus, P2 is a PE-morphism in Lact(N) It is clear that the
components of pl and of p2 are uniquely detc r-inedo All
that is left to showit is the uniqueness of C. In particular,
we must show that zo[f0l] is unique. However, this follows
from the fact that (f,l) must be an act rmor:phism and from,
consequently, the co-miutativity of the above C.icer'?.
Thus, the factorization is unioque. This completes the
proof of this proposition.

(3.18). -rr Ute see that 3.17 is just a restat ement

of 3.6, so 3.6 is proved.
(3.19). Po.c'oooit o.p.- The product functor T:EnsxEnIs-->
Ens has a left adjoint F4, where F4(A) = (A,A) and F4(f)
= (f,f).
PEoof. We will prove this using the front adjunction
theorem. For the remainder of this proof, we denote F4 as F.
Define DA:A-->AxA by DA(a) = (aa),.if A is non-enpty.
If A is the empty set, then let DA -- A. If f:A-->B, then
we have the following: [(fxf)DA (a) = (f1rf)(aa)

(f(a),f(a)) = (DBof)(a). Thus, D:I---- F is a natural
transformation Let f:A---(C(E) (=C E). Let pS:CxE---C
and p2:CCE---E be the usual projection functions. Then

(pofsP2of): (A:A)--->(C,E) is a morphism in EnsxEns.
T(pl1 fp2_f) = (p10f)x(p2f). L((P- fx(p of))DA ](a) =
((piof) x(p2of))(aa) = (p (f(a)),p2(f(a))) = f(a,). Thus,
1 f1 '2 f)DA = fI Suppose (uv)oDA = f. Then if a is
in A; (uxv)(a a) = f(a)s so f(a) = (u(a) v(a)). Thus;

(Plof)(a) = u(a) and (P2.f)(a) = v(a), so p of = u and
P2of = v. Hence, (plf,p2 f) is uir.iue, By the front
adjunction theorem, -.w then have that F is the left adjoint
of T.
(3.20). ProDosition. The functor U -= To(U2% U )U1
of 3.5 has as left adjoi nt the functor F -,-o (F2xF )oF
L 2 3 4
where FI, F2, F 9 and F4 are respectively the left adjoints

of UL, U2, U3 eundT.
Proof. The left adjoints exist by 3.5 and by 3.20,
and by 3,6. W'e refer only to the hypotheses of 3.5. The
rest is straightforward and is left to the reader.

(3.21). ,'?os,:to.~_ Let U_ be as in 3.5. Suppose
f is a regular epimrorphism in Sgp(~ ) and g is a regular
epimorphism in i implies fog is an epimrc:phism in M. Then
U reflects regular epimorphisci'C
Proof. .Lt (f'g) (Ri~,z)-->(SNzN) be a morphism
in Lact(,). Suppose U(fg) is a regular epinorphisn in
Sgp(N)x!. say U(f,g) is coeq((a,b),(a',b')) -where (a,b),
(abt):(TL)-->(RiII). By proposition 3.12, (1, (1,z o(11))):
F(R,~M)-->(B,Fz,) is a norphism in Lact(N). By 3.1i7 F(a;b):
F(T,L)--,P(R, ) is also a morphism in L.act(N\). By 3.13
and by the proofs of 3.15 and 3.17, we have F(a,b) = (ah)
where h = (u ob zo(al )o(10)(uMob))), where z is as described
in 3l11, Thus, (1, (,z ))o(ab):'(TL)-- -RM,).
(1, (19il))oI(ab) = (1, (i,z. )o(ah) = (a, (i,M)h). We
wish to express (.,z )oh in a Cimpler form. (1,z-,)hou
(1z )ou ob = lb = b. (Iz2)h)ohouT = (1, z)ozo(a1)
(10(ui ob)) = (l,z )ozo(a,(u MOb)) = (,z ))oZo(@u u)- (ab)
= (1, ,i)ou (1., 1,p l) (1Lo).d"-o1 (10u-) o(a@b.b) = z.o(1,p01)
o(rlc<). u- (ab) = z, (1. l.p1)u o .. (aob) = z-..- (aSb) -
z-(aeb). Thus, by the definition of coprocduct, we have
(l,zH )h = (b,z Io(a@b)). Hence, (a, (b,z, (aob))):F(T,L)-->
(P.BIIzi) is a i-c-'_'Il:: in Lct(N). Similarly,
(a', (b',z o (a' ') ) ) :F(T,L.)--() ,MT z ) is a mor phi n in
Lact(H). In tho above, p is the muLltiplication on R.
foa = foa'. g'e(b,zN(ago))u = gob = gob =
g(b',z .(a'QbP))-ur. g.(b,z,..(ab))tu--. = gzi.o (aGo) =
ll L u TL, Mi
zN (f-)o(ab) = zN((fa)o('f b)) ,oz ((foa) C)()-b'))

ziN*(f03) (a ? ) gz o(a"'bI ) = G -(b o~, '(aROb' ) )ucoiL

Thus, from the definition of coproduct, we have go(bz.o(aob))
Sgo(bez (a~@b~)). Tais implies (fg) (a, (bz i(b)))

(f, )(a'(bOz o(a19b'))). Suppose (uIv):(RNz) ---
(W,K,?K) in lact() so that v (u;v)(a (b,z (aOb))) = (uv)
o(a', (b'M o(a'@b))). Then, in the same manner as for
(fg). wes obtain uoa = uoa' and vcb = vrbl. Then there
exists a unique morphism (u*,vi) in Sgp(T)xN so that (,v) =
(u'v'0)o(fg). Since U1 is faithful, uniqueness in Sgp(N)YM
implies uniqueness in Lact(N). Thus, it suffices to prove

(u',v:) is a morphism in Lact(N) in order to prove that

(fg) is the coequalizer of (a,(bz,, (aBb))) and of the
morphism (a, (bqzo(a"b'))) which proves that U1 reflects
regular epinorphisms. In the following iag--.-* portions

(1), (2), and (3) and the outer rectangle coin:ute:

BZ __I___-------------___
N. zMM,
u (2) v
fes z
(1) WOK--- (3)

uS 0--U W V ,-
ThusE we have zzo(u'^s)o(,.-j) = v'ozNo (f@g). Our hypothesis
then implics that f is an epimorphiim so z (u'-ov) =
v..ozNo Consequently, (u, ,v-) is a morphism in Lact (N),
This completes the proof.

(3o22). Proposition. Suppose H and Sgp(N) have regular
epinorphism, monomorpohism factorization of morphirrZas and
that (4) in the hypotheses of 3.5 holds. Then UI of 3.5

preserves regular epi~orphisms. In fact, U1 raps cxtremal
.... c~ U1 C c o x rema3.

epimorphisms onto regular epimorphisms.
Pr-oof. Since a rc_ i.ar epimorphismr is an extremal
epimorphism, w-e mny prove only that U1 maps extremal epi-
morphisms onto regular epi]orphisms. Let (fg):(RFM,zq )
-- SN,zN) be an extremal epinorphism in Lact(N). Then

we have vob = g and uoa = f, where a and b are regular
epimorphisms in respectively S3p(N) and M and u and v are
monomorphisms respectively in Sgp(N) and M. Let dom v = B
and dom u = A. Then (fg) is a morphisn in Lact(N) implies

goZe = z N(fOg) so vobozg = zNO(uv) (adb). Theng by hy-
pothesis, there exists z :AOB--B3 so that bz Z = z B(ab)
and vzg = zN (uv). Let iu be the seiAgroup multiplication
on A. We have the follo;-.ing diagravn:

S (S)-----------------> SN
t du (uitv) (2)

C4A@C(A@3) 10ZB

(ss ()u_, )(


S(4) o(5)te sc v (uv. ially ) cor(AA)

z B
mAOB------t--S n

Then (1) commutes by the naturality of o. Also, (2), (3),
and ) commute since voz z No(uiv). Finally, (5) cO-e I
mutes since u is a morphism in S-p(N), T')n -e have that

vozBo(ul0l)o V= vZB (1B) Since v is a monomorphismi
this implies zB (:!) c = zBO(0zB). Thus, (A,B,zB) is
an object in Lact(I)o Then we have (a;b) is a norphism
in Lact(N). Also, (u,v) is a monomorphisr in Lact(Ni),
But sinse (f, ) is an extreal eplmorphism, (fg) = (u,v)
o (a,b) implies (u,v) is an isomorphism in Lact(N) so Ul(uv)
is an isonorphica in Sgp(T)xRM, iut Ul(f,s) = U1(uv)oU,(a,b)
and Ul(a,b) is a regular epimorphism in Sgp())xMl since
a and b are regular epimorphisms respectively in Sgp(N)
and ... Consequently9 Ul(fg) is a regular cpiiorphism in
Sgp(N)x;i so U1 preserves regular epimorphisms.

(3.23). ..roositio0 U1 of 3.5 reflects congruence
Proof, Suppose the following is a diagram in Lact (N):

(PQz ) P 2 (R;z

(P292) (f)s)

(,I, (------S, N zN)
Denote U1(P,Q,zQ) as (PQ) and U (p1 q1) as (pi, 1), and
so forth. Suppose the folloulng is a pullback square in
Sgp ()x .:
(P, 9Q)-- >( 1M)

(P25q2) (f(g)

( R )------ ->(s, N) .
Suppose that the foll.ouus C-.rra.mi L con mu.bes in -eact(N):

_, ,ZQ.s)--- ------- ------- ,(RH,!i, zi~i)

(P() (f,g))

1 2
(R,, Ii,z )

Then we also have (fsg) = (p.) "
Hence, there exists a unique (p,q): (P,Q' -->(P,Q) such

that, in Sgp()x, (pi,q)o(p,q) = (p,), for i = 1,2.
Since U1 is faithful, unimquness in S"p(")xi ir-plies unique-.
ness in Lact(N). Thus, it suffices to show that (p,q):
(P'eQOsZq )---(PQsz ) is a morphisn in Lact(N). Since

(ql'q2) is the pullback in 1- of g -:lth itself and since
goq : ZQ: = goqCozQ ,e have that there exists a unique
r:PF~Q-->Q so that qo r = O!ozQ for i = 1,2. Ho-ever,
qioqZQ. = qO! ,, i 1= 12. Also, q oz (pq) = z (p
o(pq.) = z( o(peq) = qoz ,. Thus, qoz2, = r = zQ (p^).
Consequ- --'-. (pi,):(', Q ,Z: )-- (PQoz ) is a morphism
in Lact(). ".s, U reflects congruence relations.

(3.24). P oz.-siion. Let (A,U) and (B,V) be algebraic
categories so that neither U nor V assumes the empty set
as a value. Then (;B, TTo(UxV)) is an algebraic category,
where VT is as in 3.19.
Proof. If F and G are respectively the left adjoints
of U and V and if H denotes the left ad. joint of T (see 3.19),
then it is easy to see that (FxG)oH is the left adjoint of
I-(UxV). The r, -'inug properties are easily verified

(3.25). 4P : Proposition 3.24 holds ,with
"el. "bz'ric" replac by varietyt al. "

?roof. The proof is straightfor.a:'d and is left to the


(3.26). Proof of 3.5. This is immediate from propo-
sitions 3.6 and 3.21 through 3.25 and from the fact that

since Ssp(N) ead .I are algebraic, they have regular epi-D

morpnism, nononorphism factorization of morphisms (see [Il).

(3.27). Cooll3r5 to 3.5. H, the category of all
modules over all rings, is a varietal category.

Proof. Let N = (Ab69,) wThere denotes the tensor

product over the integers and o is the usual associativity

transformation. Then it is well known that N is a multi-

plicative category without unit. Also, it is well kno:.m

that for any abelian group E, OE and EO_ preserve finite

(in fact, arbitrary) coproducts. ( For details about the

tensor product, the reader may sec C6].)

It -s easy to see that Sgp(N) is isomorphic to R and

tht. Lact(!) is isomorphic to Ri. Since isomorphisms pre-

serve all the properties concerned, we will check in LM
the hypotheses to 3.5.

(1) Let f be a regular epimorphism in R and g be

a regular epimorphism in Ab. Then f and g are onto functions

so f6g is an onto function, Hence, f0g is an epimorphism,

( For background ma.terial on R, see [7] .)

(2) It is well knon.m that Ab and R. with their forget-

ful functors are varietal categories and that their forget-

ful functors never assume the empty set as a value,

(3) Let (ftg):(R,K,. )--(S,N,-:,-) be a morphism in RM.
It is easy to short that the following is a congruence relation

wherc pi and qi are the natural. projections, i = 1,2, and
where wee use the induced module multiplication on the group

(mm'i (m) = n m= mR)}.m

((O.Tr')lf(r) = f(r')}, (mn') g(m) = gda')},* IRM,*)

(P2 q2) (fg)

(R, ) ---(S N, )
The details are left to the reader.
Now we construct coequalizerso Let (f,g) and (f',g)
taking (RiM,*) into (S,N,-) be two mcrphisms in i Let
N' = g g(m)-ge (m)I mleGk. Let I be the ideal of S generated
by &f(r)-f'I(r) rc}. It is easy to see that p:S-->S/I is
the coequalizer of f and fY in RH where p is the natural
projection. There is an induced module multiplication
for (S/i 5/(N',+IT+SN' )), Let q:,N-->N/(1 +IN+SN' ) be the
natural L!.p. By def nition., of the .induced multiplication,
(psq) is a morphism in R. It is clear that (pq)o(fg)
(pvq)o(fSg ). Suppose (k,t).(f,.) 1= (kt)O("P,), where
(k,t): (S,N,-)---(T,L,). Since p = coeq(f,'), there exists
a unique ring morphism k* so thai k"op = k. Define t*(q(n))
= t(n). To see that t-: is well-defined, suppose x is in
Nc, IN, or SN'. If x is in N', then x g(m)-g' (m) for
some m, so t(x) = t(g(m)-g' (m)) = t(g(m))-t(gr (m)) = 0.
For each r in R, t((f(r).-f(r)n) = k(f(r)-.f(r))t(n)
O't(n) = 0. Since I is generated by (f(r)-f' (r) rE ,
t(x) = 0, for each x in IN. t(s(g()-g(n'))) =
k(s)t(g(m)-g()) = k(s).0 =0. Thus. for each x in SN',

t(x) = 0. Hence, t* is well-defined. It is clear that
t* is a morphism in Ab an it is left to the reader to verify

that (k*,t*) is a morphis-i 1l. Since p and q are onto,

(k1:t*) is unique Thus, (p,q) is the coequalizer of (f,g)

and (f-,g').

(4) Suppose' a and b are regular epimorphisms respec-

tively in R and Abo Also, assume that v is a monomorphism

in Ab. Then a b, b, nd ab are onto and v is one-to-one.

Suppose the following diagram in Ab commutes:
a~b UBV
A'B' a.b M03 Au3v A" B"

z B, z B
b v

Define zB :AO-->3 as follows. For x in AB, asb is onto

implies that there exists x' in A'OB1 so that (a@b)(x') = x.

Define zB(x) = (bozB,)(x'), It is left to the reader to

show that zB is -ell-defined, is a morphism in Ab, and!

that the required commutativity holds.

Thus, all the hypotheses of 3.5 are satisfied. Hence,

by 3.5;, is a varietal category.

(3.28). G- erali.io's and Fnrth-r Results. We have
obtained a generalization of 3.5 which allows us to deduce

that certain subcategories of Lact(N) are variety] categories.

Due to time pressures, we are unable to include the details.

However, we would like to list a number of well kno-r cat-

egories t:hich are varietal. The proofs of these assertations

are along the same lines as the proof that RIM is varietal.

(3.29). Va-ietj. C .- copes, The following categories

are vartital:

(a) the category of all not necessarily unital modules
over all rings having an identity, ere ring morphiss

preserve identies and. morphisms of the category are analo-

gous to mor-phisis in

(b) the category of all monoids acting on pointed

sets, not nece:- --ily in a unital fashion but so that if

(M,(X,x),-) is an object in the category, then 1.x = x,

where morphiism in the category are analogous to morphisms

in RI. anY

(c) the category of all compact, Hausdorff monoids

acting on compact, Hausdorff spaces analogously as in (b).

It is our hope to publish at a later date a complete proof

of 3.29.

(3.30) Ca .'--.: Whch A- Ar:ost Varietal. Let C
be either the category of all co pact acts (see 1.17) or

the cp-'- cf all s- groups acting on sets. Let U:

C---insxEns be the forgetful functor. Then U satisfies

properties A2 A3, and V in definition 3,2. Furthermore ~

C has congruence relations and coequalizers.

Proof U has a l.eot adjoint by 3.6, We hope that

the recan.iier of this proof will be published at a later

date. DIi to time pressures, it is not given now.


1. H. Herrl.lch, -.-tr.n ca- zoeies:;i- an a orati

gpor8'D.,c to appear.
2. E. M. Norris, S, structure thlcrns for to olozical

machimnjs, Univer-sity of Florida, 1969.

3. B. litche!lls Theory oi cf a- ores, Acad eic Press


4. De Husscoller-,; Fib:re buindies, MIcGraT:-ill Boo

Company, 1966.

5. S. IMacL.~cnec C-t ,Ical al c 7-., ba, -NSF.L Advanced. Science
Seminar Lectures at EBo,.oin College, 1969.

6. C. W. Curtis and I. Reiner, RLo''ratJ.son tteo" rr.

of fiite rmo-. -o. I' `r '- R S Inters ci'ce

P.blishiers 1962.

7< K, Koss 033, i '' ." 1 O Urnivorsity of Florida,



Stephe.n Jacikson :~ -caTel was born September 21, 1945,

in Plant City, Flori.da. In June, 1963, he graduated from

Plant City Senior High School. In June, 1966, he received

the Bachelor of aArts in mathematics from the University of

South Florida0 In August, 1967, he received the Master o

Arts in mnthenatics from the University of South Florida.

From Septemibera 1967, until the present time, he has studied

at the Universit-ly of Florida toward the C.l :' c- of Doctor

of Philosophy. He has held throughout his stay at the

University of Florida a NASA Tralneeship, During the surer

of 1969, he attended an I". Advanced Science Seminar ian

Category Thoo-ry at Eo;-:doin Colle e in iMaine. He is a monb-r

of the ir '"i. n I-aths',atical Society.

This disse:rtation ivras prepared under the direction of

the cha rnai. of. *th cr-.d.idate s supervisory conmaitte and

has been approved by all inebers of that comnuittes, It was

submitted to the Dean of the Colle-e of Arts and Sciences

and to the Graduate Council: and w;as approved as partial

fulf ilrlmnt of the rqu.r-nents for the degree of Doctor

of Philosophy.

Augiust, 1970

D Collee oT'ts and Sciences

an. a z ,1;.t e cSchool
SupC..vi a. c:y Co&ii:';;lee:

Cha5. iiL -1c;-


University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs