# Grothendieck spectral sequence

In mathematics, in the field of homological algebra, the Grothendieck spectral sequence, introduced in Tôhoku paper, is a spectral sequence that computes the derived functors of the composition of two functors $G\circ F$ , from knowledge of the derived functors of F and G.

If $F:{\mathcal {A}}\to {\mathcal {B}}$ and $G:{\mathcal {B}}\to {\mathcal {C}}$ are two additive and left exact functors between abelian categories such that $F$ takes F-acyclic objects (e.g., injective objects) to $G$ -acyclic objects and if ${\mathcal {B}}$ has enough injectives, then there is a spectral sequence for each object $A$ of ${\mathcal {A}}$ that admits an F-acyclic resolution:

$E_{2}^{pq}=({\rm {R}}^{p}G\circ {\rm {R}}^{q}F)(A)\Longrightarrow {\rm {R}}^{p+q}(G\circ F)(A).$ Many spectral sequences in algebraic geometry are instances of the Grothendieck spectral sequence, for example the Leray spectral sequence.

The exact sequence of low degrees reads

0 → R1G(FA) → R1(GF)(A) → G(R1F(A)) → R2G(FA) → R2(GF)(A).

## Examples

### The Leray spectral sequence

If $X$ and $Y$ are topological spaces, let

${\mathcal {A}}=\mathbf {Ab} (X)$ and ${\mathcal {B}}=\mathbf {Ab} (Y)$ be the category of sheaves of abelian groups on X and Y, respectively and
${\mathcal {C}}=\mathbf {Ab}$ be the category of abelian groups.

For a continuous map

$f:X\to Y$ there is the (left-exact) direct image functor

$f_{*}:\mathbf {Ab} (X)\to \mathbf {Ab} (Y)$ .

We also have the global section functors

$\Gamma _{X}:\mathbf {Ab} (X)\to \mathbf {Ab}$ ,

and

$\Gamma _{Y}:\mathbf {Ab} (Y)\to \mathbf {Ab} .$ Then since

$\Gamma _{Y}\circ f_{*}=\Gamma _{X}$ and the functors $f_{*}$ and $\Gamma _{Y}$ satisfy the hypotheses (since the direct image functor has an exact left adjoint $f^{-1}$ , pushforwards of injectives are injective and in particular acyclic for the global section functor), the sequence in this case becomes:

$H^{p}(Y,{\rm {R}}^{q}f_{*}{\mathcal {F}})\implies H^{p+q}(X,{\mathcal {F}})$ for a sheaf ${\mathcal {F}}$ of abelian groups on $X$ , and this is exactly the Leray spectral sequence.

### Local-to-global Ext spectral sequence

There is a spectral sequence relating the global Ext and the sheaf Ext: let F, G be sheaves of modules over a ringed space $(X,{\mathcal {O}})$ ; e.g., a scheme. Then

$E_{2}^{p,q}=\operatorname {H} ^{p}(X;{\mathcal {E}}xt_{\mathcal {O}}^{q}(F,G))\Rightarrow \operatorname {Ext} _{\mathcal {O}}^{p+q}(F,G).$ This is an instance of the Grothendieck spectral sequence: indeed,

$R^{p}\Gamma (X,-)=\operatorname {H} ^{p}(X,-)$ , $R^{q}{\mathcal {H}}om_{\mathcal {O}}(F,-)={\mathcal {E}}xt_{\mathcal {O}}^{q}(F,-)$ and $R^{n}\Gamma (X,{\mathcal {H}}om_{\mathcal {O}}(F,-))=\operatorname {Ext} _{\mathcal {O}}^{n}(F,-)$ .

Moreover, ${\mathcal {H}}om_{\mathcal {O}}(F,-)$ sends injective ${\mathcal {O}}$ -modules to flasque sheaves, which are $\Gamma (X,-)$ -acyclic. Hence, the hypothesis is satisfied.

## Derivation

We shall use the following lemma:

Lemma — If K is an injective complex in an abelian category C such that the kernels of the differentials are injective objects, then for each n,

$H^{n}(K^{\bullet })$ is an injective object and for any left-exact additive functor G on C,

$H^{n}(G(K^{\bullet }))=G(H^{n}(K^{\bullet })).$ Proof: Let $Z^{n},B^{n+1}$ be the kernel and the image of $d:K^{n}\to K^{n+1}$ . We have

$0\to Z^{n}\to K^{n}{\overset {d}{\to }}B^{n+1}\to 0,$ which splits. This implies each $B^{n+1}$ is injective. Next we look at

$0\to B^{n}\to Z^{n}\to H^{n}(K^{\bullet })\to 0.$ It splits, which implies the first part of the lemma, as well as the exactness of

$0\to G(B^{n})\to G(Z^{n})\to G(H^{n}(K^{\bullet }))\to 0.$ Similarly we have (using the earlier splitting):

$0\to G(Z^{n})\to G(K^{n}){\overset {G(d)}{\to }}G(B^{n+1})\to 0.$ The second part now follows. $\square$ We now construct a spectral sequence. Let $A^{0}\to A^{1}\to \cdots$ be an F-acyclic resolution of A. Writing $\phi ^{p}$ for $F(A^{p})\to F(A^{p+1})$ , we have:

$0\to \operatorname {ker} \phi ^{p}\to F(A^{p}){\overset {\phi ^{p}}{\to }}\operatorname {im} \phi ^{p}\to 0.$ Take injective resolutions $J^{0}\to J^{1}\to \cdots$ and $K^{0}\to K^{1}\to \cdots$ of the first and the third nonzero terms. By the horseshoe lemma, their direct sum $I^{p,\bullet }=J\oplus K$ is an injective resolution of $F(A^{p})$ . Hence, we found an injective resolution of the complex:

$0\to F(A^{\bullet })\to I^{\bullet ,0}\to I^{\bullet ,1}\to \cdots .$ such that each row $I^{0,q}\to I^{1,q}\to \cdots$ satisfies the hypothesis of the lemma (cf. the Cartan–Eilenberg resolution.)

Now, the double complex $E_{0}^{p,q}=G(I^{p,q})$ gives rise to two spectral sequences, horizontal and vertical, which we are now going to examine. On the one hand, by definition,

${}^{\prime \prime }E_{1}^{p,q}=H^{q}(G(I^{p,\bullet }))=R^{q}G(F(A^{p}))$ ,

which is always zero unless q = 0 since $F(A^{p})$ is G-acyclic by hypothesis. Hence, ${}^{\prime \prime }E_{2}^{n}=R^{n}(G\circ F)(A)$ and ${}^{\prime \prime }E_{2}={}^{\prime \prime }E_{\infty }$ . On the other hand, by the definition and the lemma,

${}^{\prime }E_{1}^{p,q}=H^{q}(G(I^{\bullet ,p}))=G(H^{q}(I^{\bullet ,p})).$ Since $H^{q}(I^{\bullet ,0})\to H^{q}(I^{\bullet ,1})\to \cdots$ is an injective resolution of $H^{q}(F(A^{\bullet }))=R^{q}F(A)$ (it is a resolution since its cohomology is trivial),

${}^{\prime }E_{2}^{p,q}=R^{p}G(R^{q}F(A)).$ Since ${}^{\prime }E_{r}$ and ${}^{\prime \prime }E_{r}$ have the same limiting term, the proof is complete. $\square$ 