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GX
XF U
v
v
vv
vv
XF Uvv
vv XF U О· XО·
vv
vv
zvv XF U  
ОѕF U
o G XF U
XF Ur XF U F U
r
vv
rr
vv
rr
vv
rr
vv
rr vz
z rrr vv
vv
rr
vv
rr
6 zvv
Z
ПЂU is also a coequalizer of ОѕF U and XF U , if U preserves coequalizers. Moreover П•: X
G (X, Оѕ)О¦U is compatible with Оѕ and ПЂU , because ОѕП• = ОѕВ·XО·В·ПЂU = XF U О·В·ОѕF U В·ПЂU =
Л‡
XF U О·В·XF U В·ПЂU = ПЂU . Thus П• is an isomorphism in A. Trivially, if a map of T-algebras
is an isomorphism in the underlying category, then it is an isomorphism of T-algebras (its
inverse also respects T-structures). In other words, the functor U T : AT G A reп¬‚ects
isomorphisms. This proves (2).
G Y is obtained from the identity map of Y О¦ via the above 1-1
Л‡
The counit П€: Y О¦О¦
correspondences, and is uniquely determined by its appearance in the following diagram,
the top line of which is a coequalizer.

G
Y UF
G Y О¦О¦
ПЂ Л‡
G Y UF
Y UF UF v
vv
Y UF
vv
vv
vv
Y
vv П€
vv
 zvvv
Y
11
TRIPLES, ALGEBRAS AND COHOMOLOGY

We proved above that the T-structure of an algebra is a coequalizer in A. (Recall that
Y U is the T-structure of Y О¦.) If U reп¬‚ects coequalizers, then Y is a coequalizer of
Y U F and Y U F , and П€ is an isomorphism.
If U preserves coequalizers, both ПЂU and Y U are coequalizers, hence П€U is an iso-
morphism. If U reп¬‚ects isomorphisms, so is П€. This proves the remark pertaining to (3 ),
and completes the proof of the theorem.
Remarks on the functor О¦. It would be interesting to know an example of a functor
G AT with B having arbitrary limits (limit = projective limit, in our terminol-
О¦: B
Л‡
ogy) wherein no left adjoint О¦ exists. Counterexamples apparently exist when B is not
complete.
Whether an adjoint or not, О¦ preserves all an adjoint should preserve, for example,
limits and algebraic objects. Indeed, in the commutative diagram
О¦ GB
AT c

cc

cc 
c1  U
UT
A

U , being an adjoint, preserves the property involved, and one п¬Ѓnds that U T reп¬‚ects the
property. We saw above that U T reп¬‚ects isomorphisms. Similarly, one can show that U T
reп¬‚ects all other limits. In В§2 we will need the fact, and will then prove in detail, that О¦
preserves several kinds of algebraic objects.
О¦ can also be given an interpretation in terms of вЂњstructureвЂќ and вЂњsemanticsвЂќ. Let
Ad(A) be the category of adjoint pairs over A, that is, pairs F U where U has A as
range, with functors which commute with the right adjoints (like О¦ itself) as maps. Let
G T being a natural transformation
Trip(A) denote the category of triples in A, a map S
of the functors commuting with the units and multiplications, and let Trip(A)в€— denote
the dual category. Then functors
Л‡ G
Пѓ
Trip(A)в€—
Ad(A) o Пѓ

exist, because adjoint pairs give rise to triples and triples give rise to categories AT with
G Trip(A)в€— , and the
adjoint free and underlying object functors. In fact, Пѓ Л‡ Пѓ, ПѓЛ‡
Пѓ
G Пѓ Пѓ. All of this is proved in [Eilenberg
unit of this adjointness is precisely О¦: Ad(A) Л‡
& Moore (1965a)], and is reminiscent of the structure-semantics situation studied by
[Lawvere (1963)] in the case of algebraic categories and by [Linton (1966)] in the case of
equational categories. We should remark that by means of a more elaborate construction
G A (B
the domain of Пѓ can be extended to the category whose objects are functors B
Л‡
variable) which are вЂњsmallвЂќ in an appropriate sense but do not need to have left adjoints.
It is useful to think of the triple T induced by an adjoint pair of functors as a structural
invariant of the adjoint pair. Various properties of the adjoint pair only depend on T and
the functor О¦ plays a role in setting up the relevant isomorphisms. We shall see this
illustrated in the next section in the case of cohomology.
12 JONATHAN MOCK BECK

2. Cohomology
Adjoint functors, it is now well known, lead to cohomology ([Eilenberg & Moore (1965a),
Godement (1958), Mac Lane (1963)]вЂ”or to homotopy [Huber (1961)]). If

GB GA
F U
A (F U)

is an adjoint pair, objects of the form AF в€€ |B| are regarded as вЂњfreeвЂќ relative to the
underlying object functor U . The counit

GX
X
XU F

is intuitively the п¬Ѓrst step of a functorial free resolution of any object X в€€ |B|. By
iterating U F one extends X to a free simplicial resolution of X, and deп¬Ѓnes derived
functors as usual in homological algebra. Here we only consider the simplest case, that
of deп¬Ѓning cohomology groups

n в‰Ґ 0,
H n (X, Y ),

of an object X в€€ |B| with coeп¬ѓcients in an abelian group object Y в€€ |B|, relative to
G A (having a left adjoint). Tripleableness
the given underlying object functor U : B
of F U will not play any appreciable role until we discuss special properties of the
cohomology in В§3. We now recall the details of the construction of the cohomology groups.
Some of the terms used are clariп¬Ѓed in the proof of Theorem 2, which summarizes the
main properties the cohomology possesses.
G B.
Let (G, , Оґ) be the cotriple in B induced by F U ; thus G = U F and : G
The following simplicial object in B is called the standard (free simplicial) resolution of
the object X:
o
XG oo
0
XG2 oo
Xo В·В·В· o XGn o XGn+1 o
0 i
В·В·В·
1

We abbreviate this by XGв€— if necessary. Here XGn+1 is the term of degree n, and the
G XGn is Gi Gnв€’i , 0 в‰¤ i в‰¤ n. X itself is in dimension в€’1 and
face operator i : XGn+1
the last 0 augmenting the simplicial object into X is just . The simplicial identities

i в‰¤ j,
= j+1 i ,
ij

G G2 induces degeneracy operators but these will play no
can easily be veriп¬Ѓed. (Оґ: G
role in our theory.)
G Y . If Y is an abelian
An n-cochain of X with coeп¬ѓcients in Y is a map XGn+1
group object in the category B, the n-cochains (XGn+1 , Y ) form an abelian group, and
G (XGn+1 , Y ).
the face operators i induce abelian group maps (X i , Y ): (XGn , Y )
Thus
d1 d2 dn+1
G (XG2 , Y )
G (XG, Y ) G В·В·В· G (XGn+1 , Y ) G (XGn+2 , Y ) G В·В·В·
0
13
TRIPLES, ALGEBRAS AND COHOMOLOGY

is a cochain complex of abelian groups, where dn+1 = (в€’1)i (X i , Y ), 0 в‰¤ i в‰¤ n + 1,
dd = 0 because of the simplicial identities, and the augmentation term has been dropped.
To distinguish it from another complex that will be introduced later, this complex is called
the homogeneous complex. We deп¬Ѓne

n в‰Ґ 0,
H n (X, Y ),
G X in B and Y
as the n-th cohomology group of this complex. Obviously maps X
G Y in the category of abelian group objects in B induce maps

G H n (X , Y ), G H n (X, Y )
H n (X, Y ) H n (X, Y )

and the cohomology is functorial in the usual way.
I do not know how to characterize the cohomology theory H(X, Y ) = (H n (X, Y )),
n в‰Ґ 0, axiomatically. However, acyclicity of free objects and the exact cohomology
sequence are properties that can easily be established:

THEOREM 2.
(AF, Y ), n = 0,
n
H (AF, Y ) =
0, n > 0В·
GY GY GY G 0 is a U -exact sequence of abelian group objects in
If 0
B, then there is an exact cohomology sequence
В· В· В· lВ€В€В€В€В€В€В€В€
В€В€В€В€В€d
В€В€В€В€В€
В€В€В€В€В€
В€В€В€
G H n (X, Y ) G H n (X, Y )
n
H (X, Y ) lВ€В€В€В€
В€В€В€В€В€ d
В€В€В€В€В€
В€В€В€В€В€
В€В€
G H (X, Y ) G В·В·В· G H nв€’1 (X, Y )
0
0

Proof. For the п¬Ѓrst part, we notice that the standard resolution of a free object AF has
a simplicial contracting homotopy
sв€’1 s0 sn
G AF G2
G AF G G В·В·В· G AF Gn+1 G AF Gn+2 G В·В·В·
AF

given by sn = AО·F Gn+1 (see [Huber (1961), p. 248], for example; this homotopy satisп¬Ѓes
the relations sn 0 = identity, sn i = iв€’1 snв€’1 , 1 в‰¤ i в‰¤ n + 1). If we let tn = (sn , Y ), then

tв€’1 t0 tn
(AF G2 , Y ) o
0o (AF, Y ) o (AF G, Y ) o В·В·В· o (AF Gn+2 , Y ) o В·В·В·

is a contracting homotopy of the augmented cochain complex, that is, we have dt + td =
identity. Hence H 0 (AF, Y ) is the term of degree в€’1 and all higher cohomology vanishes.
For the second part we must explain the concept of U -exactness; this requires recalling
some facts about abelian group objects in categories (see [Eckmann & Hilton (1962)] for
14 JONATHAN MOCK BECK

a fuller treatment). Y is an abelian group object in B if the hom set (B, Y ) has an abelian
group structure for every object B in the category B, and naturality holds in that every
G (B , Y ) is an abelian group map, where B G B in B. Y
induced map (B, Y )
G Y is a map of abelian group objects in B if it is a map in B and every induced (B, Y )
G (B, Y ) is an abelian group map. The abelian group objects form a category AbB,
G B. Of course, one can deп¬Ѓne in the same way
with an obvious forgetful functor AbB
other types of algebraic objects in categories, such as objects with base points, nonabelian
groups, rings, in fact models for any algebraic theory [Lawvere (1963)], and we will need
some of these categories later. However, what general theory we will use is adequately
illustrated by the case of abelian groups. The following lemma can be paraphrased by
saying that adjoints preserve abelian group objects:

LEMMA 1. Let A o U B be a functor which has a left adjoint, and let Y в€€ |AbB|. Then
there exists a unique abelian group structure on Y U в€€ |A| such that
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