1

Chain Complexes

1.1 Complexes of R-Modules

Homological algebra is a tool used in several branches of mathematics: alge-braic topology, group theory, commutative ring theory, and algebraic geometrycome to mind. It arose in the late 1800s in the following manner. Let f and gbe matrices whose product is zero. If g . II = 0 for some column vector V, say,of length n, we cannot always write u = f . u. This failure is measured by thedefect

d = II - rank(f) - rank(g).

In modern language, f and g represent linear maps

with gf = 0, and d is the dimension of the homology module

H = ker(s>lfW).

In the first part of this century, Poincare and other algebraic topologistsutilized these concepts in their attempts to describe n-dimensional holes insimplicial complexes. Gradually people noticed that vector space could bereplaced by R-module for any ring R.

This being said, we fix an associative ring R and begin again in the categorymod-R of right R-modules. Given an R-module homomorphism f: A + B,one is immediately led to study the kernel ker(f), cokernel coker(f), andimage im(f) of f. Given another map g: B -+ C, we can form the sequence

(*I ALBg'C.

2 Chain Complexes

We say that such a sequence is exact (at B) if ker(g) = im(f). This impliesin particular that the composite gf: A + C is zero, and finally brings ourattention to sequences (*) such that gf = 0.

Definition 1.1.1 A chain complex C. of R-modules is a family [Cn)nEz ofR-modules, together with R-module maps d = d,,: C, -+ C,_I such that eachcomposite d o d: C, -+ C,,-2 is zero. The maps d,, are called the dzxerentialsof C.. The kernel of d,, is the module of n-cycles of C,, denoted 2, = Z, (C.).The image of d,,+l: C,,+l + C, is the module of n-boundaries of C,, denotedB, = B,,(C,). Because d o d = 0, we have

for all n. The nth homology module of C, is the subquotient H,,(C.) = Z,lB,of C,. Because the dot in C, is annoying, we will often write C for C..

Exercise 1.1.1 Set C, = Z/8 for n >_ 0 and C, = 0 for n < 0; for n > 0let d,, send x(mod 8) to 4x(mod 8). Show that C. is a chain complex ofZ/8-modules and compute its homology modules.

There is a category Ch(mod-R) of chain complexes of (right) R-modules.The objects are, of course, chain complexes. A morphism u: C. + D, is achain complex map, that is, a family of R-module homomorphisms un: C, +D, commuting with d in the sense that un-Id, = dn_lu,. That is, such thatthe following diagram commutes

d d d d. . . + Cn+l - c, + c,-I - ...

d d d d. . . + Dntl - D, - D,-1 - . .

Exercise 1.1.2 Show that a morphism U: C, + D. of chain complexes sendsboundaries to boundaries and cycles to cycles, hence maps H,, (C.) + H,, (D.).Prove that each H, is a functor from Ch(mod-R) to mod-R.

Exercise 1.1.3 (Split exact sequences of vector spaces) Choose vector spaces(Bn, HnJnEz over a field, and set C, = B, @ Hn @ B,_l. Show that theprojection-inclusions C, + B,_l c C,_ 1 make (C,) into a chain complex,and that every chain complex of vector spaces is isomorphic to a complex ofthis form.

1.1 Complexes of R-Modules 3

Exercise 1.1.4 Show that {HomR(A, C,)) forms a chain complex of abeliangroups for every R-module A and every R-module chain complex C. TakingA = Z,, show that if H,(HomR(Z,, C)) = 0, then H,(C) = 0. Is the conversetrue?

Definition 1.1.2 A morphism C. -+ D. of chain complexes is called a quasi-isomorphism (Bourbaki uses homologism) if the maps H,(C.) -+ H,(D.) areall isomorphisms.

Exercise 1.1.5 Show that the following are equivalent for every C. :

1. C. is enact, that is, exact at every C,.2. C, is acyclic, that is, H,(C.) = 0 for all 12.3. The map 0 + C, is a quasi-isomorphism, where 0” is the complex of

zero modules and zero maps.

The following variant notation is obtained by reindexing with superscripts:C” = C-,. A cochain complex C. of R-modules is a family {C } of R-modules, together with maps dn: Cn -+ Cn+’ such that d o d = 0. Z (C.) =ker(dn) is the module of n-cocycles, B (C,) = im(d - ) C Cn is the mod-ule of n-coboundaries, and the subquotient H (C.) = Z /B” of C” is the nthcohomology module of C.. Morphisms and quasi-isomorphisms of cochaincomplexes are defined exactly as for chain complexes.

A chain complex C, is called bounded if almost all the C, are zero; ifC, = 0 unless a ( n 5 b, we say that the complex has amplitude in [a, b]. Acomplex C, is bounded above (resp. bounded below) if there is a bound b (resp.a) such that C, = 0 for all n > b (resp. n < a). The bounded (resp. boundedabove, resp. bounded below) chain complexes form full subcategories of Ch= Ch(R-mod) that are denoted Chb, Ch_ and Ch+, respectively. The sub-category Ch,o of non-negative complexes C. (C, = 0 for all n < 0) will be_important in Chapter 8.

Similarly, a cochain complex C. is called bounded above if the chain com-plex C, (C, = C?) is bounded below, that is, if Cn = 0 for all large n; C.is bounded below if C, is bounded above, and bounded if C. is bounded.The categories of bounded (resp. bounded above, resp. bounded below, resp.non-negative) cochain complexes are denoted Chb, Ch-, Chf, and Ch ,respectively.

Exercise 1.1.6 (Homology of a graph) Let I be a finite graph with V vertices(~1, . . , VV) and E edges (el, . . . , e&. If we orient the edges, we can form theincidence matrix of the graph. This is a V x E matrix whose (ij) entry is fl

4 Chain Complexes

if the edge ej starts at vi, - 1 if ej ends at ui, and 0 otherwise. Let CO be thefree R-module on the vertices, Cl the free R-module on the edges, C, = 0if n # 0, 1, and d: Cl -+ Co be the incidence matrix. If r is connected (i.e.,we can get from IJO to every other vertex by tracing a path with edges), showthat Ho(C) and HI(C) are free R-modules of dimensions 1 and V - E - 1respectively. (The number V - E - 1 is the number of circuits of the graph.)Hint: Choose basis (~0, vt - vu, . . . , Z)V - uo} for Co, and use a path from vuto ui to find an element of Ct mapping to vi - uu.

Application 1.1.3 (Simplicial homology) Here is a topological applicationwe shall discuss more in Chapter 8. Let K be a geometric simplicial complex,such as a triangulated polyhedron, and let Kk (0 5 k 5 n) denote the set ofk-dimensional simplices of K. Each k-simplex has k + 1 faces, which areordered if the set Ko of vertices is ordered (do so!), so we obtain k + 1 setmaps &: Rk + &_t (0 ( i 5 k). The simplicial chain complex of K withcoefficients in R is the chain complex C., formed as follows. Let Ck be the freeR-module on the set Kk; set Ck = 0 unless 0 5 k 5 n. The set maps & yieldk + 1 module maps Ck -+ C&t, which we also call &; their alternating sumd = c(- l) & is the map Ck + Ck-1 in the chain complex C.. To see that C,is a chain complex, we need to prove the algebraic assertion that d o d = 0.This translates into the geometric fact that each (k - 2)-dimensional simplexcontained in a fixed k-simplex o of K lies on exactly two faces of U. Thehomology of the chain complex C, is called the simplicial homology of K withcoefficients in R. This simplicial approach to homology was used in the firstpart of this century, before the advent of singular homology.

Exercise 1.1.7 (Tetrahedron) The tetrahedron T is a surface with 4 ver-tices, 6 edges, and 4 2-dimensional faces. Thus its homology is the homol-ogy of a chain complex 0 -+ R4 + R6 + R4 + 0. Write down the matricesin this complex and verify computationally that Hz(T) g Ho(T) &Z R andHI(T) = 0.

Application 1.1.4 (Singular homology) Let X be a topological space, andlet Sk = Sk(X) be the free R-module on the set of continuous maps fromthe standard k-simplex Ak to X. Restriction to the ith face of .& (0 5 i 5 k)transforms a map & + X into a map Ak_1 + X, and induces an R-modulehomomorphism ai from Sk to ,!$_I. The alternating sums d = x(-l) & (fromSk to Sk-t) assemble to form a chain complex

1.2 Operations on Chain Complexes 5

called the singular chain complex of X. The nrh homology module of S.(X) iscalled the nth singular homology of X (with coefficients in R) and is writtenZ&(X; R). If X is a geometric simplicial complex, then the obvious inclusionC,(X) -+ S.(X) is a quasi-isomorphism, so the simplicial and singular homol-ogy modules of X are isomorphic. The interested reader may find details inany standard book on algebraic topology.

1.2 Operations on Chain Complexes

The main point of this section will be that chain complexes form an abeliancategory. First we need to recall what an abelian category is. A reference forthese definitions is [MacCW].

A category A is called an Ab-category if every horn-set HomA(A, B) inA is given the structure of an abelian group in such a way that compositiondistributes over addition. In particular, given a diagram in A of the form

we have h(g + g )f = hgf + hg f in Hom(A, D). The category Ch is an Ab-category because we can add chain maps degreewise; if (f,,) and {gn} are chainmaps from C, to D., their sum is the family of maps { fn + gn}.

An additive functor F: 23 -+ A between Ab-categories t3 and d is a functorsuch that each Homa(B , B) -+ HomA(FB , FB) is a group homomorphism.

An additive category is an Ab-category A with a zero object (i.e., an ob-ject that is initial and terminal) and a product A x B for every pair A, B ofobjects in A. This structure is enough to make finite products the same as fi-nite coproducts. The zero object in Ch is the complex 0” of zero modulesand maps. Given a family (A,] of complexes of R-modules, the product lTA,and coproduct (direct sum) @A, exist in Ch and are defined degreewise: thedifferentials are the maps

respectively. These suffice to make Ch into an additive category.

Exercise 1.2.1 Show that direct sum and direct product commute with ho-mology, that is, that @H,,(A,) G %(@A,) and lXH,(A,) 2 H,(lTA,) forall n.

6 Chain Complexes

Here are some important constructions on chain complexes. A chain com-plex B is called a subcomplex of C if each B, is a submodule of C, and thedifferential on B is the restriction of the differential on C, that is, when theinclusions i, : B, C C, constitute a chain map B + C. In this case we canassemble the quotient modules C,/ B, into a chain complex

. ..--+ Cn+l/B,+l 5 C,IB, A C,_l/B,_l 5 ...denoted C/B and called the quotient complex. If f: B + C is a chain map, thekernels {ker(f,,)} assemble to form a subcomplex of B denoted ker(f>, andthe cokemels (coker(f,)} assemble to form a quotient complex of C denotedcoker(f).

Definition 1.2.1 In any additive category A, a kernel of a morphism f: B +C is defined to be a map i: A + B such that fi = 0 and that is universal withrespect to this property. Dually, a cokernel of f is a map e: C -+ D, whichis universal with respect to having ef = 0. In A, a map i: A + B is manicif ig = 0 implies g = 0 for every map g: A + A, and a map e: C + D isan epi if he = 0 implies h = 0 for every map h: D -+ D . (The definition ofmanic and epi in a non-abelian category is slightly different; see A.1 in theAppendix.) It is easy to see that every kernel is manic and that every cokemelis an epi (exercise!).

Exercise 1.2.2 In the additive category A = R-mod, show that:

I. The notions of kernels, monies, and monomorphisms are the same.2. The notions of cokemels, epis, and epimorphisms are also the same.

Exercise 1.2.3 Suppose that A = Ch and f is a chain map. Show that thecomplex ker( f) is a kernel of f and that coker( f) is a cokemel of f .

Definition 1.2.2 An abelian category is an additive category A such that

1. every map in A has a kernel and cokemel.2. every manic in A is the kernel of its cokemel.3. every epi in A is the cokernel of its kernel.

The prototype abelian category is the category mod-R of R-modules. Inany abelian category the image im(f) of a map f: B -+ C is the subobjectker(coker f) of C; in the category of R-modules, im(f) = {f(b) : b E B).Every map f factors as

1.2 Operations on Chain Complexes

B 5 im(.f) $ C

with e an epimorphism and m a monomorphism. A sequence

of maps in A is called exact (at B) if ker(g) = im(f).A subcategory t? of A is called an abelian subcategory if it is abelian, and

an exact sequence in B is also exact in A.If A is any abelian category, we can repeat the discussion of section 1.1

to define chain complexes and chain maps in d-just replace mod-R by A!These form an additive category Ch(d), and homology becomes a functorfrom this category to A. In the sequel we will merely write Ch for Ch(d)when A is understood.

Theorem 1.2.3 The category Ch = Ch(d) of chain complexes is an abeliancategory.

Proof Condition 1 was exercise 1.2.3 above. If f: B + C is a chain map, Iclaim that f is manic iff each B, + C,, is manic, that is, B is isomorphic to asubcomplex of C. This follows from the fact that the composite ker(f) + Cis zero, so if f is manic, then ker(f) = 0. So if f is manic, it is isomorphic tothe kernel of C -+ C/B. Similarly, f is an epi iff each B, + C, is an epi, thatis, C is isomorphic to the cokemel of the chain map ker(f) + B,. 0

Exercise 1.2.4 Show that a sequence 0 -+ A, -+ B. --+ C. -+ 0 of chain com-plexes is exact in Ch just in case each sequence 0 -+ A,, -+ B, --+ C, + 0 isexact in A.

Clearly we can iterate this construction and talk about chain complexes ofchain complexes; these are usually called double complexes.

Example 1.2.4 A double complex (or bicomplex) in A is a family {C,,,) ofobjects of A, together with maps

dh: C,,, --+ C,_l,, a n d dU: C,,, + C,,,-I

such that dh o dh = d” o d = d dh + dhd” = 0. It is useful to picture thebicomplex C.. as a lattice

8 Chain Complexes. . . . . . . . .

1 1 1dh dh. . . - c,-1,,+1 - Cp,,+l - cp+1,,+1 - ..’

dUL dU1 4.

. . . -CP-Lq L cp,, E Cp+l,q - . .

a d L. 4

dh dh. . - cp-14-l - Cp,,-1 - c,+1,,_1 t- . . .

L 1 I. . . . . . . . .

in which the maps d” go horizontally, the maps d go vertically, and eachsquare anticommutes. Each row C,, and each column C,, is a chain complex.

We say that a double complex C is bounded if C has only finitely manynonzero terms along each diagonal line p + 4 = n, for example, if C is con-centrated in the first quadrant of the plane (ajrst quadrant double complex).

Sign Trick 1.2.5 Because of the anticommutivity, the maps d are not mapsin Ch, but chain maps f*s from C*, to C*,q_t can be defined by introducingf signs:

fp,4 = (-1)pd;,4: Cp,q + Cp,q-1.

Using this sign trick, we can identify the category of double complexes withthe category Ch(Ch) of chain complexes in the abelian category Ch.

Total Complexes 1.2.6 To see why the anticommutative condition d dh +dhd” = 0 is useful, define the total complexes Tot(C) = Tot (C) and Tot@(C)by

T o t ( C ) , = fl C,,, and To t@(C) , = @ C,,,.p+q=n p+q=n

The formula d = dh + d defines maps (check this!)

d : Tot (C), + Tot (C),_1 and d : Tot (C), -+ Tot@(C),_t

such that d o d = 0, making Tot (C) and Tot@(C) into chain complexes. Notethat Tot@(C) = Tot (C) if C is bounded, and especially if C is a first quadrantdouble complex. The difference between Tot (C) and Tot@(C) will becomeapparent in Chapter 5 when we discuss spectral sequences.

1.2 Operations on Chain Complexes 9

Remark Tot (C) and Tot@(C) do not exist in all abelian categories; theydon t exist when A is the category of all finite abelian groups. We say thatan abelian category is complete if all infinite direct products exist (and soTot exists) and that it is cocomplete if all infinite direct sums exist (and soTot@ exists). Both these axioms hold in R-mod and in the category of chaincomplexes of R-modules.

Exercise 1.2.5 Give an elementary proof that Tot(C) is acyclic whenever Cis a bounded double complex with exact rows (or exact columns). We will seelater that this result follows from the Acyclic Assembly Lemma 2.7.3. It alsofollows from a spectral sequence argument (see Definition 5.62 and exercise5.6.4).

Exercise 1.2.6 Give examples of (1) a second quadrant double complex Cwith exact columns such that Tot (C) is acyclic but Tot@(C) is not; (2) asecond quadrant double complex C with exact rows such that Tot@(C) isacyclic but Tot (C) is not; and (3) a double complex (in the entire plane) forwhich every row and every column is exact, yet neither Tot (C) nor Tot@(C)is acyclic.

Truncations 1.2.7 If C is a chain complex and n is an integer, we let r?,,Cdenote the subcomplex of C defined by

0 ifi <n(t>nC)i = Z, if i = R

Ci ifi >n.

Clearly Hi(t>,C) = 0 for i < n and Hi(r,,C) = Hi(C) for i > n. The com-plex t+C is called the (good) truncation of C below n, and the quotientcomplex t<,,C = C/(t,,C) is called the (good) truncation of C above n;Hi(t<,C) is Hi(C) for 7 < n and 0 for i > n.

Some less useful variants are the brutal truncations u<,,C and oznC =C/(%l C). By definition, (o<,C)i is Ci if i < n and 0 if i > n. These havethe advantage of being easier to describe but the disadvantage of introducingthe homology group H,,(a&) = C,/B,.

Translation 1.2.8 Shifting indices, or translation, is another useful operationwe can perform on chain and cochain complexes. If C is a complex and p aninteger, we form a new complex C[p] as follows:

c[p] , = Cnfp (resp. C[pY = C - )

10 Chain Complexes

with differential (-1)Pd. We call C[p] the pth translate of C. The way toremember the shift is that the degree 0 part of C[p] is C,. The sign conventionis designed to simplify notation later on. Note that translation shifts homology:

fL(C[pl) = &fp(C) (rev. ff (C[~l) = ff -p(C>h

We make translation into a functor by shifting indices on chain maps. That is,if f: C + D is a chain map, then f[p] is the chain map given by the formula

f[Pln = fn+p (resp. f[p]” = fnPP).

Exercise 1.2.7 If C is a complex, show that there are exact sequences ofcomplexes:

0 ---+ Z ( C ) --+ c -5 B(C)[-l] ---_, 0 ;

0 - H(C) - C/B(C) A Z(C)[-11 - H(C)[-11 - 0 .

Exercise 1.2.8 (Mapping cone) Let f: B + C be a morphism of chain com-plexes. Form a double chain complex D out of f by thinking of f as a chaincomplex in Ch and using the sign trick, putting B [-- l] in the row 9 = 1 andC in the row q = 0. Thinking of C and B[- 11 as double complexes in theobvious way, show that there is a short exact sequence of double complexes

0 - C ----+ D - B[-I] + 0.

The total complex of D is cone(f ), the mapping cone (see section 1.5) ofa map f , which differs from f only by some f signs and is isomorphicto f.

1.3 Long Exact Sequences

It is time to unveil the feature that makes chain complexes so special from acomputational viewpoint: the existence of long exact sequences.

Theorem 1.3.1 Let 0 --+ A. L B. & C, + 0 be a short exact sequence ofchain complexes. Then there are natural maps a: H,(C) -+ H,_l (A), calledconnecting homomorphisms, such that

. L f&+,(C) A K,(A) z H,(B)-II-, H,(C)A Hn_l(A)L

is an exact sequence.

1.3 Long Exact Sequences 11

Similarly, if 0 -+ A. L B. -% C. -+ 0 is a short exact sequence ofcochain complexes, there are natural maps a: Hn(C) -+ Hnfl (A) and a longexact sequence

. . &H - (C)& H"(A)L H"(B)& H"(C)& H"+'(A)L

Exercise 1.3.1 Let 0 -+ A + B -+ C + 0 be a short exact sequence of com-plexes. Show that if two of the three complexes A, B, C are exact, then so isthe third.

Exercise 1.3.2 (3 x 3 lemma) Suppose giben a commutative diagram

0 + A + B/' + C" + 0

1 L 10 0 0

in an abelian category, such that every column is exact. Show the following:

1. If the bottom two rows are exact, so is the top row.2. If the top two rows are exact, so is the bottom row.3. If the top and bottom rows are exact, and the composite A -+ C is zero,

the middle row is also exact.

Hint: Show the remaining row is a complex, and apply exercise 1.3.1.

The key tool in constructing the connecting homomorphism a is our nextresult, the Snake Lemma. We will not print the proof in these notes, becauseit is best done visually. In fact, a clear proof is given by Jill Clayburgh at thebeginning of the movie It s My Turn (Rastar-Martin Elfand Studios, 1980). Asan exercise in diagram chasing of elements, the student should find a proof(but privately-keep the proof to yourself!).

Snake Lemma 1.3.2 Consider a commutative diagram of R-modules of theform

12 Chain Complexes

A + B -_, C’ --+ 0

O-A&B-C.

If the rows are exact, there is an exact sequence

ker(f) --f ker(g) --f ker(h) a\ coker(f) -+ coker(g) -+ coker(h)

with 8 defined by the formula.

a(c ) = i- gp- (c ), c E ker(h).

Moreover, if A + B is manic, then so is ker(f) + ker(g), and if B + C isonto, then so is coker(f) --, coker(g).

d R

Etymology The term snake comes from the following visual mnemonic:

ker(f) --+ ker(g) + ker(h) - - - - -.,

. - - - - f coker(f) + coker(g) --+ coker(h).

Remark The Snake Lemma also holds in an arbitrary abelian category C. Tosee this, let A be the smallest abelian subcategory of C containing the ob-jects and morphisms of the diagram. Since d has a set of objects, the Freyd-Mitchell Embedding Theorem (see 1.6.1) gives an exact, fully faithful embed-ding of A into R-mod for some ring R. Since a exists in R-mod, it exists inA and hence in C. Similarly, exactness in R-mod implies exactness in A andhence in C,

1.3 Long Exact Sequences

Exercise 1.3.3 (5-Lemma) In any commutative diagram

A + B + C + D’ --+ E’

al= bL= c-i dls +

A---+B-C---+D-+E

13

with exact rows in any abelian category, show that if a, b, d, and e are isomor-phisms, then c is also an isomorphism. More precisely, show that if b and dare manic and a is an epi, then c is manic. Dually, show that if b and d areepis and e is manic, then c is an epi.

We now proceed to the construction of the connecting homomorphism a ofTheorem 1.3.1 associated to a short exact sequence

O+A-+B+C+O

of chain complexes. From the Snake Lemma and the diagram

P P ;

0 - Z,A ---+ Z,B - Z,C

I L 1

O+ A,, ----+ B, + C, -0

4 4 4.

0 + A,_1 --+ B,_l + C,_l - 0

I I I

An-1 B,-1 cn-1

dA,- dB,---0dG

I L I

0 0 0

we see that the rows are exact in the commutative diagram

A, & GdA- dBn+l- -0

flfl dG+l

4 dl dlf

0 ---+ Z,pl(A) + Z,-l(b) A Z,-](C).

14 Chain Complexes

The kernel of the left vertical is H,,(A), and its cokernel is H,_l (A). Thereforethe Snake Lemma yields an exact sequence

H,(A)f HJB,A H,(C)& H,-l(A)+ f&-l(B)-+ H,-l(C).

The long exact sequence 1.3.1 is obtained by pasting these sequences together.

Addendum 1.3.3 When one computes with modules, it is useful to be able topush elements around. By decoding the above proof, we obtain the followingformula for the connecting homomorphism: Let z E H,,(C), and represent it bya cycle c E C,. Lift the cycle to b E B, and apply d. The element db of B,_lactually belongs to the submodule Z,_t (A) and represents i!)(z) E H,_l(A).

We shall now explain what we mean by the naturality of a. There is acategory S whose objects are short exact sequences of chain complexes (say,in an abelian category C). Commutative diagrams

O-A-+B--+C-+O

(*) L I I

0 4 A -_, B -_, C + 0

give the morphisms in S (from the top row to the bottom row). Similarly, thereis a category L of long exact sequences in C.

Proposition 1.3.4 The long exact sequence is a functor from S to C. That is,for every short exact sequence there is a long exact sequence, and for everymap (*) of short exact sequences there is a commutative ladder diagram

... A H , ( A ) ---F H , ( B ) - H , ( C ) a\ H,_I(A)--+ ...

I I I 1

. . . 5 H,(A ) --f H,(B ) - H,(C ) A H,-,(A )-+ . . . .

Proof All we have to do is establish the ladder diagram. Since each H,, is afunctor, the left two squares commute. Using the Embedding Theorem 1.6.1,we may assume C = mod-R in order to prove that the right square commutes.Given z E H,(C), represented by c E C,, its image z E H,(C ) is representedby the image of c. If b E B, lifts c, its image in BL lifts c . Therefore by 1.3.3a (z ) E H,_I (A ) is represented by the image of db, that is, by the image of arepresentative of a(z), so i3(z > is the image of a(z). 0

1.4 Chain Homotopies 15

Remark 1.3.5 The data of the long exact sequence is sometimes organizedinto the mnemonic shape

H,(A) - H*(B)6 Jl

H*(C)

This is called an exact triangle for obvious reasons. This mnemonic shapeis responsible for the term triangulated category, which we will discuss inChapter 10. The category K of chain equivalence classes of complexes andmaps (see exercise 1.4.5 in the next section) is an example of a triangulatedcategory.

Exercise 1.3.4 Consider the boundaries-cycles exact sequence 0 + Z +C + B(- 1) + 0 associated to a chain complex C (exercise 1.2.7). Show thatthe corresponding long exact sequence of homology breaks up into short exactsequences.

Exercise 1.3.5 Let f be a morphism of chain complexes. Show that if ker(f)and coker(f) are acyclic, then f is a quasi-isomorphism. Is the converse true?

Exercise 1.3.6 Let 0 -+ A + B + C + 0 be a short exact sequence of dou-ble complexes of modules. Show that there is a short exact sequence of totalcomplexes, and conclude that if Tot(C) is acyclic, then Tot(A) + Tot(B) is aquasi-isomorphism.

1.4 Chain Homotopies

The ideas in this section and the next are motivated by homotopy theory intopology. We begin with a discussion of a special case of historical impor-tance. If C is any chain complex of vector spaces over a field, we can alwayschoose vector space decompositions:

C, = Z, 63 B;, B:, % C,/Z, = d(C,,) = B,_l;

Z, = B, @I H;, H; z Z,/B, = H,,(C).

Therefore we can form the compositions

C,, + Z, + B, % B;+l E C,,+l

16 Chain Complexes

to get splitting maps sn: C,, -+ Cn+t, such that d = dsd. The compositions dsand sd are projections from C, onto B, and BA, respectively, so the sum ds +sd is an endomorphism of C, whose kernel HL is isomorphic to the homologyZ&(C). The kernel (and cokemel!) of ds + sd is the trivial homology complexH,(C). Evidently both chain maps H,(C) + C and C -+ H*(C) are quasi-isomorphisms. Moreover, C is an exact sequence if and only if ds + sd is theidentity map.

Over an arbitrary ring R, it is not always possible to split chain complexeslike this, so we give a name to this notion.

Definition 1.4.1 A complex C is called split ifthere are maps s,,: C, + C,,+lsuch that d = dsd. The maps s, are called the splitting maps. If in addition Cis acyclic (exact as a sequence), we say that C is split exact.

Example 1.4.2 Let R = Z or Z/4, and let C be the complex

This complex is acyclic but not split exact. There is no map s such that ds + sdis the identity map, nor is there any direct sum decomposition C,, 2 Z, @ BL.

Exercise 1.4.1 The previous example shows that even an acyclic chain com-plex of free R-modules need not be split exact.

1. Show that acyclic bounded below chain complexes of free R-modulesare always split exact.

2. Show that an acyclic chain complex of finitely generated free abeliangroups is always split exact, even when it is not bounded below.

Exercise 1.4.2 Let C be a chain complex, with boundaries B,, and cycles Z,in C,. Show that C is split if and only if there are R-module decompositionsC, 2 Z, @ BA and Z, = B, @ HA. Show that C is split exact iff Hi = 0.

Now suppose that we are given two chain complexes C and D, togetherwith randomly chosen maps sn: C, + D,+ 1. Let f , , be the map from C, to D,defined by the formula fn = dn+ls, + sn_ld, .

d dCnfl ---, G --f G-1

SLA fl SkY

D,,+I + D, + D,-Id d

1.4 Chain Homotopies

Dropping the subscripts for clarity, we compute

17

df = d(ds + sd) = dsd = (ds + sd)d = f d.

Thus f = ds + sd is a chain map from C to D.

Definition 1.4.3 We say that a chain map f: C + D is null homotopic ifthere are maps sn: C, -+ Dn+l such that f = ds + sd. The maps {sn} arecalled a chain contraction of f.

Exercise 1.4.3 Show that C is a split exact chain complex if and only if theidentity map on C is null homotopic.

The chain contraction construction gives us an easy way to proliferate chainmaps: if g: C + D is any chain map, so is g + (sd + ds) for any choice ofmaps sn. However, g + (sd + ds) is not very different from g, in a sense thatwe shall now explain.

Definition 1.4.4 We say that two chain maps f and g from C to D are chainho&otopic if their difference f - g is null homotopic, that is, if

f -g=sd+ds.

The maps (s,} are called a chain homotopy from f to g. Finally, we say thatf: C -+ D is a chain homotopy equivalence (Bourbaki uses homotopism) ifthere is a map g: D + C such that gf and fg are chain homotopic to therespective identity maps of C and D.

Remark This terminology comes from topology via the following observa-tion. A map f between two topological spaces X and Y induces a mapf*: S(X) + S(Y) between the corresponding singular chain complexes. Itturns out that if f is topologically null homotopic (resp. a homotopy equiv-alence), then the chain map f* is null homotopic (resp. a chain homotopyequivalence), and if two maps f and g are topologically homotopic, then f*and g* are chain homotopic.

Lemma 1.4.5 If f : C -+ D is null homotopic, then every map f*: H,(C) +H,,(D) is zero. Zf f and g are chain homotopic, then they induce the samemaps H,(C) + H,(D).

Proof It is enough to prove the first assertion, so suppose that f = ds + sd.Every element of H,(C) is represented by an n-cycle x. But then f(x) =d(sx). That is, f(x) is an n-boundary in D. As such, f(x) represents 0 inK(D). 0

18 Chain Complexes

Exercise 1.4.4 Consider the homology H,(C) of C as a chain complex withzero differentials. Show that if the complex C is split, then there is a chainhomotopy equivalence between C and H,(C). Give an example in which theconverse fails.

Exercise 1.4.5 In this exercise we shall show that the chain homotopy classesof maps form a quotient category K of the category Ch of all chain complexes.The homology functors H,, on Ch will factor through the quotient functorCh-+K.

Show that chain homotopy equivalence is an equivalence relation onthe set of all chain maps from C to D. Let HomK(C, D) denote theequivalence classes of such maps. Show that HomK(C, D) is an abeliangroup.Let f and g be chain homotopic maps from C to D. If U: B + C andu: D + E are chain maps, show that vf u and vgu are chain homotopic.Deduce that there is a category K whose objects are chain complexes andwhose morphisms are given in (1).Let fu, fl, go, and gl be chain maps from C to D such that fi is chainhomotopic to gi (i = 1, 2). Show that fu + ft is chain homotopic togo + gl. Deduce that K is an additive category, and that Ch + K is anadditive functor.Is K an abelian category? Explain.

1.5 Mapping Cones and Cylinders

1.5.1 Let f: B, + C, be a map of chain complexes. The mapping cone off is the chain complex cone(f) whose degree n part is B,_l @ C,,. In orderto match other sign conventions, the differential in cone(f) is given by theformula

d(B, c) = (--d(B), d(c) - f(B)), (b E B,-1, c E C,).

That is, the differential is given by the matrix

B,_l _\ Bn_2

[--4” :,I: @ \- 63 .

G - G - 1-c