Derived Functors

\(\delta\)-functor

We previously proved that, given any short exact sequence in \(\Ch(\mathcal{A})\)

\[0\longrightarrow A_\bullet\longrightarrow B_\bullet\longrightarrow C_\bullet\longrightarrow 0\]

we can construct the long exact sequence

\[\cdots\rightarrow H_n(A)\rightarrow H_n(B)\rightarrow H_n(C)\rightarrow H_{n-1}(A)\rightarrow \cdots\]

The most essential part of this proof was defining the connecting map \(\delta\), and we generalize this process to define the following.

Definition 1 Let two abelian categories \(\mathcal{A},\mathcal{B}\) be given. Then a homological \(\delta\)-functor from \(\mathcal{A}\) to \(\mathcal{B}\) refers to additive functors \(T_n:\mathcal{A}\rightarrow\mathcal{B}\) (\(n\geq 0\)), together with morphisms \(\delta_n:T_n(C)\rightarrow T_{n-1}(A)\) defined for each short exact sequence

\[0\longrightarrow A\longrightarrow B\longrightarrow C\longrightarrow 0\]

For \(n<0\), we regard all \(T_n\) as \(0\). These satisfy the following conditions.

1. (Long exact sequence) The sequence

   \[\cdots\longrightarrow T_{n+1}(C)\overset{\delta}{\longrightarrow}T_n(A)\longrightarrow T_n(B)\longrightarrow T_n(C)\overset{\delta}{\longrightarrow}T_{n-1}(A)\longrightarrow \cdots\]

   is exact.

2. (Naturality) Given a homomorphism between short exact sequences

   the following diagram

   commutes.

Rewriting the above definition with \(T^n\) and \(\delta^n:T^n(C)\rightarrow T^{n+1}(A)\), we obtain the definition of a cohomological \(\delta\)-functor. Since we agreed that for \(n<0\), both \(T_n\) and \(T^n\) are \(0\), the first condition for a homological \(\delta\)-functor means in particular that

\[\cdots\longrightarrow T_0(A)\longrightarrow T_0(B)\longrightarrow T_0(C)\longrightarrow0\longrightarrow 0\longrightarrow\cdots,\]

that is, \(T_0\) is a right exact functor. Similarly, the first condition for a cohomological \(\delta\)-functor makes \(T^0\) a left exact functor.

Also, the second condition of a \(\delta\)-functor, naturality, means that when we consider the two functors from the collection of short exact sequences \(\mathbf{S}(\mathcal{A})\) to \(\mathcal{B}\), \(T_i(C)\) and \(T_{i-1}(A)\), \(\delta_i\) is a natural transformation between them.

As always, since the case of a cohomological \(\delta\)-functor can be easily derived from a homological \(\delta\)-functor, we will consider only homological \(\delta\)-functors from now on.

Definition 2 Let two \(\delta\)-functors \(S,T\) be given. Then a morphism from \(S\) to \(T\), \(S\rightarrow T\), refers to a collection of natural transformations from \(S_n\) to \(T_n\) that commute with \(\delta\).

That is, for every short exact sequence

\[0\longrightarrow A\longrightarrow B\longrightarrow C\longrightarrow 0\]

it is a collection of \(\alpha_n:S_n\Rightarrow T_n\) making the following diagram commute.

Definition 3 A \(\delta\)-functor \(T\) is called a universal \(\delta\)-functor if, whenever a \(\delta\)-functor \(S\) and a natural transformation \(\alpha_0:S_0\rightarrow T_0\) are given, there exists a unique morphism of \(\delta\)-functors \((\alpha_n:S_n\Rightarrow T_n)\) extending it.

Derived Functors

Between two abelian categories \(\mathcal{A},\mathcal{B}\), consider a right exact functor \(F: \mathcal{A}\rightarrow \mathcal{B}\). Then \(F\) does not preserve left exactness. For example, if \(F\) is a covariant functor, then even when the short exact sequence

\[0 \rightarrow A_1 \rightarrow A_2 \rightarrow A_3 \rightarrow 0\]

is given, only the exactness of the sequence

\[F(A_1) \rightarrow F(A_2) \rightarrow F(A_3)\rightarrow 0\]

is preserved. Likewise, a left exact functor does not preserve right exactness.

The philosophy of derived functors is to compensate for the information lost on the left or the right in this way by using infinitely many terms. That is, for a right exact functor \(F\), our goal is to find, with \(T_0=F\), homological \(\delta\)-functors, and similarly for a left exact functor, our goal is to find cohomological \(\delta\)-functors.

Definition 4 Suppose that a right exact functor \(F:\mathcal{A}\rightarrow \mathcal{B}\) is given, and that \(\mathcal{A}\) has enough projectives. Then, for \(F\), the left derived functors \(L_iF\) are defined by the formula

\[(L_iF)(A)=H_i(F(P_\bullet)),\qquad\text{$P_\bullet$ a projective resolution of $A$}\]

For this definition to make sense, \(L_iF(A)\) must not depend on the choice of \(P_\bullet\) above.

Lemma 5 \(L_iF(A)\) does not depend on the choice of \(P_\bullet\) above.

Proof

Take two projective resolutions and apply §Resolutions, ⁋Theorem 6 to the identity map.

Now let us examine left derived functors in a little more detail. First, from the fact that \(F\) is right exact, we know that the sequence

\[F(P_1) \overset{Fd_1}{\longrightarrow} F(P_0) \overset{F\epsilon_0}{\longrightarrow} F(A) \longrightarrow 0\]

is exact. Therefore, we obtain

\[L_0F(A)=H_0(F(P_\bullet))=\frac{F(P_0)}{\im Fd_1}=\frac{F(P_0)}{\ker F\epsilon_0}\cong F(A)\]

To show that the \(L_\bullet F\) form a homological \(\delta\)-functor, we must first show that they are additive functors, and then construct the connecting maps \(\delta\). We divide this into two steps.

Lemma 6 The \(L_iF\) are additive functors.

Proof

First, given any \(f: A' \rightarrow A\) and projective resolutions of \(A'\) and \(A\) respectively, we can apply §Resolutions, ⁋Theorem 6 to obtain \(L_nF(f)\). Since two lifts of \(f\) are chain homotopic and the additive functor \(F\) preserves chain homotopy, \(L_nF(f)\) does not depend on the choice of lift and satisfies functoriality, and from the fact that for two morphisms \(f,g:A'\rightarrow A\) and their lifts \(f_\bullet,g_\bullet\), \(f_\bullet+g_\bullet\) is a lift of \(f+g\), we obtain additivity.

Lemma 7 The \(L_iF\) form a homological \(\delta\)-functor.

Proof

First, suppose a short exact sequence

\[0 \rightarrow A \rightarrow B \rightarrow C \rightarrow 0\]

is given. If projective resolutions \(P_\bullet\) of \(A\) and \(R_\bullet\) of \(C\) are given, then using §Resolutions, ⁋Lemma 7 we obtain a projective resolution \(Q_\bullet \rightarrow B\). On the other hand, since each \(R_n\) is projective, the sequence

\[0 \rightarrow P_n \rightarrow Q_n \rightarrow R_n \rightarrow 0\]

is split exact. From this,

\[0 \rightarrow F(P_\bullet) \rightarrow F(Q_\bullet) \rightarrow F(R_\bullet) \rightarrow 0\]

is also a short exact sequence ([Multilinear Algebra] §Hom and the Tensor Product, ⁋Proposition 1), and considering the homology sequence here, we obtain the desired connecting maps and the long exact sequence of left derived functors

\[\cdots\overset{\delta}{\longrightarrow}L_iF(A)\longrightarrow L_iF(B)\longrightarrow L_iF(C)\overset{\delta}{\longrightarrow}L_{i-1}F(A)\longrightarrow L_{i-1}F(B)\longrightarrow L_{i-1}F(C)\overset{\delta}{\longrightarrow}\cdots\]

That the information thus obtained satisfies the second condition of Definition 1 follows from §Resolutions, ⁋Theorem 6.

Moreover, they define a universal homological \(\delta\)-functor in the sense of Definition 3. We omit the proof of this.

Proposition 8 Consider an abelian category \(\mathcal{A}\) with enough projectives and any right exact functor \(F: \mathcal{A}\rightarrow \mathcal{B}\). Then the derived functors \(L_nF\) are universal \(\delta\)-functors.

Just as in the discussion above, we can also define right derived functors for a left exact functor. Its definition is the “dual” of Definition 4.

Definition 9 Let a left exact functor \(F:\mathcal{A}\rightarrow \mathcal{B}\) be given, and suppose \(\mathcal{A}\) has enough injectives. Then the right derived functors \(R^i F\) of \(F\) are defined by the formula

\[(R^iF)(A)=H^i(F(I^\bullet)),\qquad\text{$I^\bullet$ an injective resolution of $A$}\]

Then one can also show that these are universal cohomological \(\delta\)-functors. The reason we use superscripts, unlike in Definition 4, is that these are literally cohomological \(\delta\)-functors, and they arise mainly when dealing with matters related to cohomology.

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References

[Wei] C.A. Weibel. An Introduction to Homological Algebra. Cambridge Studies in Advanced Mathematics. Cambridge University Press, 1995.