Lemma 13.16.7 (Leray's acyclicity lemma). Let $F : \mathcal{A} \to \mathcal{B}$ be an additive functor between abelian categories. Let $A^\bullet $ be a bounded below complex of right $F$-acyclic objects such that $RF$ is defined at $A^\bullet $^{1}. The canonical map

\[ F(A^\bullet ) \longrightarrow RF(A^\bullet ) \]

is an isomorphism in $D^{+}(\mathcal{B})$, i.e., $A^\bullet $ computes $RF$.

**Proof.**
Let $A^\bullet $ be a bounded complex of right $F$-acyclic objects. We claim that $RF$ is defined at $A^\bullet $ and that $F(A^\bullet ) \to RF(A^\bullet )$ is an isomorphism in $D^+(\mathcal{B})$. Namely, it holds for complexes with at most one nonzero right $F$-acyclic object by Definition 13.15.3. Next, suppose that $A^ n = 0$ for $n \not\in [a, b]$. Using the “stupid” truncations we obtain a termwise split short exact sequence of complexes

\[ 0 \to \sigma _{\geq a + 1} A^\bullet \to A^\bullet \to \sigma _{\leq a} A^\bullet \to 0 \]

see Homology, Section 12.15. Thus a distinguished triangle $(\sigma _{\geq a + 1} A^\bullet , A^\bullet , \sigma _{\leq a} A^\bullet )$. By induction hypothesis $RF$ is defined for the two outer complexes and these complexes compute $RF$. Then the same is true for the middle one by Lemma 13.14.12.

Suppose that $A^\bullet $ is a bounded below complex of acyclic objects such that $RF$ is defined at $A^\bullet $. To show that $F(A^\bullet ) \to RF(A^\bullet )$ is an isomorphism in $D^{+}(\mathcal{B})$ it suffices to show that $H^ i(F(A^\bullet )) \to H^ i(RF(A^\bullet ))$ is an isomorphism for all $i$. Pick $i$. Consider the termwise split short exact sequence of complexes

\[ 0 \to \sigma _{\geq i + 2} A^\bullet \to A^\bullet \to \sigma _{\leq i + 1} A^\bullet \to 0. \]

Note that this induces a termwise split short exact sequence

\[ 0 \to \sigma _{\geq i + 2} F(A^\bullet ) \to F(A^\bullet ) \to \sigma _{\leq i + 1} F(A^\bullet ) \to 0. \]

Hence we get distinguished triangles

\[ (\sigma _{\geq i + 2} A^\bullet , A^\bullet , \sigma _{\leq i + 1} A^\bullet ) \quad \text{and}\quad (\sigma _{\geq i + 2} F(A^\bullet ), F(A^\bullet ), \sigma _{\leq i + 1} F(A^\bullet )) \]

Since $RF$ is defined at $A^\bullet $ (by assumption) and at $\sigma _{\leq i + 1}A^\bullet $ (by the first paragraph) we see that $RF$ is defined at $\sigma _{\geq i + 1}A^\bullet $ and we get a distinguished triangle

\[ (RF(\sigma _{\geq i + 2} A^\bullet ), RF(A^\bullet ), RF(\sigma _{\leq i + 1} A^\bullet )) \]

See Lemma 13.14.6. Using these distinguished triangles we obtain a map of exact sequences

\[ \xymatrix{ H^ i(\sigma _{\geq i + 2} F(A^\bullet )) \ar[r] \ar[d] & H^ i(F(A^\bullet )) \ar[r] \ar[d]^\alpha & H^ i(\sigma _{\leq i + 1} F(A^\bullet )) \ar[r] \ar[d]^\beta & H^{i + 1}(\sigma _{\geq i + 2} F(A^\bullet )) \ar[d] \\ H^ i(RF(\sigma _{\geq i + 2} A^\bullet )) \ar[r] & H^ i(RF(A^\bullet )) \ar[r] & H^ i(RF(\sigma _{\leq i + 1} A^\bullet )) \ar[r] & H^{i + 1}(RF(\sigma _{\geq i + 2} A^\bullet )) } \]

By the results of the first paragraph the map $\beta $ is an isomorphism. By inspection the objects on the upper left and the upper right are zero. Hence to finish the proof it suffices to show that $H^ i(RF(\sigma _{\geq i + 2} A^\bullet )) = 0$ and $H^{i + 1}(RF(\sigma _{\geq i + 2} A^\bullet )) = 0$. This follows immediately from Lemma 13.16.1.
$\square$

## Comments (5)

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