TheoremComplete

Grothendieck Spectral Sequence

The Grothendieck spectral sequence relates the derived functors of a composition GFG \circ F to the individual derived functors RpGR^pG and RqFR^qF. 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

Theorem7.8Grothendieck Spectral Sequence

Let F:ABF : \mathcal{A} \to \mathcal{B} and G:BCG : \mathcal{B} \to \mathcal{C} be left exact functors between abelian categories. Suppose A\mathcal{A} and B\mathcal{B} have enough injectives, and FF sends injective objects of A\mathcal{A} to GG-acyclic objects of B\mathcal{B} (i.e., RqG(F(I))=0R^qG(F(I)) = 0 for q>0q > 0 when II is injective in A\mathcal{A}). Then for every object AAA \in \mathcal{A}, there is a convergent spectral sequence:

E2p,q=RpG(RqF(A))Rp+q(GF)(A)E_2^{p,q} = R^pG(R^qF(A)) \Rightarrow R^{p+q}(G \circ F)(A)

with differentials dr:Erp,qErp+r,qr+1d_r : E_r^{p,q} \to E_r^{p+r, q-r+1}.


Proof

Proof

Step 1: Setup.

Choose an injective resolution AIA \to I^\bullet in A\mathcal{A}. Apply FF to get a complex F(I)F(I^\bullet) in B\mathcal{B}. Since GFG \circ F is left exact and II^\bullet is an injective resolution:

Rn(GF)(A)=Hn(G(F(I)))R^n(G \circ F)(A) = H^n(G(F(I^\bullet)))

Step 2: Resolve in B\mathcal{B}.

For each nn, choose an injective resolution F(In)Jn,F(I^n) \to J^{n,\bullet} in B\mathcal{B}. This gives a double complex J,J^{\bullet, \bullet} with a map F(I)J,0F(I^\bullet) \to J^{\bullet, 0}.

Step 3: Two filtrations on the total complex.

The total complex Tot(G(J,))\mathrm{Tot}(G(J^{\bullet,\bullet})) has two filtrations, giving two spectral sequences.

First filtration (rows first): For fixed qq, the row G(J,q)G(J^{\bullet, q}) computes RpGR^pG of the qq-th term. The E1E_1 page is E1p,q=RpG(F(Iq))E_1^{p,q} = R^pG(F(I^q)). By hypothesis, F(Iq)F(I^q) is GG-acyclic, so E1p,q=0E_1^{p,q} = 0 for p>0p > 0 and E10,q=G(F(Iq))E_1^{0,q} = G(F(I^q)). Hence E20,q=Hq(G(F(I)))=Rq(GF)(A)E_2^{0,q} = H^q(G(F(I^\bullet))) = R^q(G \circ F)(A) and E2p,q=0E_2^{p,q} = 0 for p>0p > 0. This spectral sequence degenerates, confirming the abutment is Rn(GF)(A)R^n(G \circ F)(A).

Second filtration (columns first): For fixed pp, the column G(Jp,)G(J^{p, \bullet}) computes derived functors of GG applied to F(Ip)F(I^p). The E1E_1 page is E1p,q=RqG(F(Ip))E_1^{p,q} = R^qG(F(I^p)). The E2E_2 page is:

E2p,q=Hp(RqG(F(I)))E_2^{p,q} = H^p(R^qG(F(I^\bullet)))

Since RqGR^qG is the qq-th cohomology of GG applied to an injective resolution, and II^\bullet is an injective resolution of AA, we get Hp(RqG(F(I)))=RpG(RqF(A))H^p(R^qG(F(I^\bullet))) = R^pG(R^qF(A)) (by a standard argument using the universal delta-functor property).

Step 4: Convergence.

Both spectral sequences converge to Hn(Tot(G(J,)))H^n(\mathrm{Tot}(G(J^{\bullet,\bullet}))). The first shows this equals Rn(GF)(A)R^n(G \circ F)(A). The second gives E2p,q=RpG(RqF(A))Rp+q(GF)(A)E_2^{p,q} = R^pG(R^qF(A)) \Rightarrow R^{p+q}(G \circ F)(A).


Examples

ExampleLeray spectral sequence

Take F=f:Sh(X)Sh(Y)F = f_* : \mathbf{Sh}(X) \to \mathbf{Sh}(Y) and G=Γ(Y,):Sh(Y)AbG = \Gamma(Y, -) : \mathbf{Sh}(Y) \to \mathbf{Ab}. Then GF=Γ(X,)G \circ F = \Gamma(X, -) and RqF=RqfR^qF = R^qf_*, RpG=Hp(Y,)R^pG = H^p(Y, -). The Grothendieck spectral sequence gives:

E2p,q=Hp(Y,RqfF)Hp+q(X,F)E_2^{p,q} = H^p(Y, R^qf_*\mathcal{F}) \Rightarrow H^{p+q}(X, \mathcal{F})

The acyclicity condition holds because ff_* preserves injective sheaves (as it has an exact left adjoint f1f^{-1}).

ExampleHochschild-Serre spectral sequence

For NGN \trianglelefteq G, take F=()N:G-Mod(G/N)-ModF = (-)^N : G\text{-}\mathbf{Mod} \to (G/N)\text{-}\mathbf{Mod} and G=()G/NG = (-)^{G/N}. Then GF=()GG \circ F = (-)^G and:

E2p,q=Hp(G/N,Hq(N,M))Hp+q(G,M)E_2^{p,q} = H^p(G/N, H^q(N, M)) \Rightarrow H^{p+q}(G, M)

The acyclicity condition holds: ()N(-)^N sends injective GG-modules to injective G/NG/N-modules.

ExampleLocal-to-global Ext

For sheaves E,F\mathcal{E}, \mathcal{F} on a scheme XX, take F=Hom(E,)F = \mathcal{H}om(\mathcal{E}, -) (the sheaf Hom) and G=Γ(X,)G = \Gamma(X, -). Then:

E2p,q=Hp(X,Extq(E,F))Extp+q(E,F)E_2^{p,q} = H^p(X, \mathcal{E}xt^q(\mathcal{E}, \mathcal{F})) \Rightarrow \mathrm{Ext}^{p+q}(\mathcal{E}, \mathcal{F})

This relates the local Ext sheaves to the global Ext groups.

ExampleLyndon-Hochschild-Serre for group extensions

For a group extension 1NGQ11 \to N \to G \to Q \to 1 (not necessarily split), the spectral sequence Hp(Q,Hq(N,M))Hp+q(G,M)H^p(Q, H^q(N, M)) \Rightarrow H^{p+q}(G, M) reduces to the five-term exact sequence:

0H1(Q,MN)infH1(G,M)resH1(N,M)Qd2H2(Q,MN)infH2(G,M)0 \to H^1(Q, M^N) \xrightarrow{\mathrm{inf}} H^1(G, M) \xrightarrow{\mathrm{res}} H^1(N, M)^Q \xrightarrow{d_2} H^2(Q, M^N) \xrightarrow{\mathrm{inf}} H^2(G, M)

ExampleCech-to-derived spectral sequence

For an open cover U\mathcal{U} of XX, there is a spectral sequence Hˇp(U,Hq(F))Hp+q(X,F)\check{H}^p(\mathcal{U}, \mathcal{H}^q(\mathcal{F})) \Rightarrow H^{p+q}(X, \mathcal{F}) where Hq\mathcal{H}^q denotes the presheaf UHq(U,F)U \mapsto H^q(U, \mathcal{F}). For a Leray cover (where all intersections are acyclic), Hq=0\mathcal{H}^q = 0 for q>0q > 0 and the spectral sequence degenerates.

ExampleComposite of pushforwards

For maps XfYgZX \xrightarrow{f} Y \xrightarrow{g} Z:

E2p,q=Rpg(RqfF)Rp+q(gf)FE_2^{p,q} = R^pg_*(R^qf_*\mathcal{F}) \Rightarrow R^{p+q}(g \circ f)_*\mathcal{F}

This computes the higher direct images of the composite from those of ff and gg individually.

ExampleDegeneration at E_2

The Grothendieck spectral sequence degenerates at E2E_2 (i.e., E2=EE_2 = E_\infty) when RqF(A)=0R^qF(A) = 0 for q0q \neq 0 (so the aisle is concentrated on one line) or when RpG=0R^pG = 0 for p0p \neq 0. In the first case, Rn(GF)(A)RnG(F(A))R^n(G \circ F)(A) \cong R^nG(F(A)). In the second, Rn(GF)(A)G(RnF(A))R^n(G \circ F)(A) \cong G(R^nF(A)).

ExampleEdge maps of Grothendieck SS

The edge maps are: Rn(GF)(A)RnG(F(A))R^n(G \circ F)(A) \to R^nG(F(A)) (the natural map from composite derived functors to GG derived at F(A)F(A)) and G(RnF(A))Rn(GF)(A)G(R^nF(A)) \to R^n(G \circ F)(A). These are isomorphisms when the spectral sequence degenerates along the corresponding edge.

ExampleAcyclicity condition

The hypothesis that FF sends injectives to GG-acyclics is essential. If FF does not preserve acyclicity, the spectral sequence still exists but with a more complicated E2E_2 page involving hyper-derived functors. In practice, the acyclicity condition is almost always satisfied in geometric situations.

ExampleDerived category interpretation

In derived categories: R(GF)RGRFR(G \circ F) \simeq RG \circ RF when FF preserves GG-acyclics. The spectral sequence arises from the t-structure filtration on RG(RF(A))RG(RF(A)). The E2E_2 page RpG(RqF(A))R^pG(R^qF(A)) is the cohomology of the cohomology, and the spectral sequence converges to the cohomology of the total complex.

ExampleFailure of the acyclicity condition

For F:ABF : \mathcal{A} \to \mathcal{B} and G:BCG : \mathcal{B} \to \mathcal{C} where FF does NOT send injectives to GG-acyclics, one can still construct a spectral sequence using Cartan-Eilenberg resolutions (double injective resolutions). The resulting spectral sequence has the same E2E_2 page but the convergence proof is more delicate.

ExampleSpectral sequence for Tor

There is a dual version for left derived functors. For right exact functors F,GF, G with FF sending projectives to GG-acyclics:

Ep,q2=LpG(LqF(A))Lp+q(GF)(A)E^2_{p,q} = L_pG(L_qF(A)) \Rightarrow L_{p+q}(G \circ F)(A)

This gives the Kunneth spectral sequence and the universal coefficient spectral sequence as special cases.


RemarkUniversality

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 FF and GG.

RemarkRelationship to derived categories

In the language of derived categories, the Grothendieck spectral sequence arises from the canonical isomorphism R(GF)RGRFR(G \circ F) \cong RG \circ RF (under the acyclicity hypothesis). The spectral sequence is a computational tool for extracting the cohomology of R(GF)(A)R(G \circ F)(A) from the cohomology of RF(A)RF(A) and the derived functors of GG.