Grothendieck Spectral Sequence
The Grothendieck spectral sequence relates the derived functors of a composition to the individual derived functors and . It is the universal machine behind the Leray spectral sequence, the Hochschild-Serre spectral sequence, the local-to-global spectral sequence for Ext, and many others.
Statement
Let and be left exact functors between abelian categories. Suppose and have enough injectives, and sends injective objects of to -acyclic objects of (i.e., for when is injective in ). Then for every object , there is a convergent spectral sequence:
with differentials .
Proof
Step 1: Setup.
Choose an injective resolution in . Apply to get a complex in . Since is left exact and is an injective resolution:
Step 2: Resolve in .
For each , choose an injective resolution in . This gives a double complex with a map .
Step 3: Two filtrations on the total complex.
The total complex has two filtrations, giving two spectral sequences.
First filtration (rows first): For fixed , the row computes of the -th term. The page is . By hypothesis, is -acyclic, so for and . Hence and for . This spectral sequence degenerates, confirming the abutment is .
Second filtration (columns first): For fixed , the column computes derived functors of applied to . The page is . The page is:
Since is the -th cohomology of applied to an injective resolution, and is an injective resolution of , we get (by a standard argument using the universal delta-functor property).
Step 4: Convergence.
Both spectral sequences converge to . The first shows this equals . The second gives .
Examples
Take and . Then and , . The Grothendieck spectral sequence gives:
The acyclicity condition holds because preserves injective sheaves (as it has an exact left adjoint ).
For , take and . Then and:
The acyclicity condition holds: sends injective -modules to injective -modules.
For sheaves on a scheme , take (the sheaf Hom) and . Then:
This relates the local Ext sheaves to the global Ext groups.
For a group extension (not necessarily split), the spectral sequence reduces to the five-term exact sequence:
For an open cover of , there is a spectral sequence where denotes the presheaf . For a Leray cover (where all intersections are acyclic), for and the spectral sequence degenerates.
For maps :
This computes the higher direct images of the composite from those of and individually.
The Grothendieck spectral sequence degenerates at (i.e., ) when for (so the aisle is concentrated on one line) or when for . In the first case, . In the second, .
The edge maps are: (the natural map from composite derived functors to derived at ) and . These are isomorphisms when the spectral sequence degenerates along the corresponding edge.
The hypothesis that sends injectives to -acyclics is essential. If does not preserve acyclicity, the spectral sequence still exists but with a more complicated page involving hyper-derived functors. In practice, the acyclicity condition is almost always satisfied in geometric situations.
In derived categories: when preserves -acyclics. The spectral sequence arises from the t-structure filtration on . The page is the cohomology of the cohomology, and the spectral sequence converges to the cohomology of the total complex.
For and where does NOT send injectives to -acyclics, one can still construct a spectral sequence using Cartan-Eilenberg resolutions (double injective resolutions). The resulting spectral sequence has the same page but the convergence proof is more delicate.
There is a dual version for left derived functors. For right exact functors with sending projectives to -acyclics:
This gives the Kunneth spectral sequence and the universal coefficient spectral sequence as special cases.
The Grothendieck spectral sequence is the "mother of all spectral sequences" in algebra. Nearly every spectral sequence in homological algebra (Leray, Hochschild-Serre, Lyndon, local-to-global, Cech-to-derived) is a special case of the Grothendieck spectral sequence for an appropriate choice of and .
In the language of derived categories, the Grothendieck spectral sequence arises from the canonical isomorphism (under the acyclicity hypothesis). The spectral sequence is a computational tool for extracting the cohomology of from the cohomology of and the derived functors of .