ProofComplete

Spectral Sequences - Key Proof

We construct the spectral sequence of a filtered complex in detail, the foundational example of spectral sequences.

Theorem7.18Spectral Sequence of Filtered Complex

Let (C,d)(C, d) be a chain complex with an increasing filtration: β‹―βŠ†Fpβˆ’1CβŠ†FpCβŠ†Fp+1CβŠ†β‹―βŠ†C\cdots \subseteq F_{p-1}C \subseteq F_pC \subseteq F_{p+1}C \subseteq \cdots \subseteq C

such that d(FpC)βŠ†FpCd(F_pC) \subseteq F_pC. Then there exists a spectral sequence with: E0p,q=FpCp+q/Fpβˆ’1Cp+qE_0^{p,q} = F_pC_{p+q} / F_{p-1}C_{p+q} E1p,q=Hp+q(FpC/Fpβˆ’1C)E_1^{p,q} = H_{p+q}(F_pC / F_{p-1}C)

converging to Hβˆ—(C)H_*(C) with filtration induced from CC.

Proof

Step 1: Define E0E_0 and d0d_0. Set: E0p,q=FpCp+q/Fpβˆ’1Cp+qE_0^{p,q} = F_pC_{p+q} / F_{p-1}C_{p+q}

The differential dd on CC induces d0d_0 on E0E_0 by passing to quotients. Since d(FpC)βŠ†FpCd(F_pC) \subseteq F_pC, this is well-defined.

Step 2: Compute E1E_1. By definition: E1p,q=Hp+q(E0p,βˆ™,d0)=Hp+q(FpC/Fpβˆ’1C)E_1^{p,q} = H_{p+q}(E_0^{p,\bullet}, d_0) = H_{p+q}(F_pC / F_{p-1}C)

This is the homology of the pp-th graded piece.

Step 3: Construct d1d_1. The differential d1:E1p,qβ†’E1pβˆ’1,qd_1: E_1^{p,q} \to E_1^{p-1,q} comes from the connecting homomorphism in the long exact sequence: β‹―β†’Hn(Fpβˆ’1C)β†’Hn(FpC)β†’Hn(FpC/Fpβˆ’1C)β†’βˆ‚Hnβˆ’1(Fpβˆ’1C)β†’β‹―\cdots \to H_n(F_{p-1}C) \to H_n(F_pC) \to H_n(F_pC/F_{p-1}C) \xrightarrow{\partial} H_{n-1}(F_{p-1}C) \to \cdots

Specifically, d1=βˆ‚:E1p,qβ†’E1pβˆ’1,qd_1 = \partial: E_1^{p,q} \to E_1^{p-1,q} where we identify: E1p,q=Hp+q(FpC/Fpβˆ’1C)E_1^{p,q} = H_{p+q}(F_pC/F_{p-1}C)

Step 4: Verify d1∘d1=0d_1 \circ d_1 = 0. This follows from the exactness of the long exact sequence: the composition of consecutive connecting homomorphisms is zero.

Step 5: Compute E2E_2. By definition: E2p,q=ker⁑(d1:E1p,qβ†’E1pβˆ’1,q)/Im(d1:E1p+1,qβ†’E1p,q)E_2^{p,q} = \ker(d_1: E_1^{p,q} \to E_1^{p-1,q}) / \text{Im}(d_1: E_1^{p+1,q} \to E_1^{p,q})

From the long exact sequence, this is related to the relative homology groups.

Step 6: Higher pages. For rβ‰₯2r \geq 2, define: Zrp,q={z∈FpCp+q:dz∈Fpβˆ’rCp+qβˆ’1}Z_r^{p,q} = \{z \in F_pC_{p+q} : dz \in F_{p-r}C_{p+q-1}\} Brp,q={b∈FpCp+q:b=dz+bβ€²Β whereΒ z∈Fp+rβˆ’1Cp+q+1,bβ€²βˆˆFpβˆ’1Cp+q}B_r^{p,q} = \{b \in F_pC_{p+q} : b = dz + b' \text{ where } z \in F_{p+r-1}C_{p+q+1}, b' \in F_{p-1}C_{p+q}\}

Then: Erp,q=(Zrp,q+Fpβˆ’1Cp+q)/(Brp,q+Fpβˆ’1Cp+q)E_r^{p,q} = (Z_r^{p,q} + F_{p-1}C_{p+q}) / (B_r^{p,q} + F_{p-1}C_{p+q})

The differential dr:Erp,qβ†’Erpβˆ’r,q+rβˆ’1d_r: E_r^{p,q} \to E_r^{p-r,q+r-1} is induced by dd on CC.

Step 7: Convergence. As rβ†’βˆžr \to \infty: E∞p,q=(Z∞p,q+Fpβˆ’1Cp+q)/(B∞p,q+Fpβˆ’1Cp+q)E_\infty^{p,q} = (Z_\infty^{p,q} + F_{p-1}C_{p+q}) / (B_\infty^{p,q} + F_{p-1}C_{p+q})

where: Z∞p,q={z∈FpCp+q:dz=0}=Z(FpC)p+q∩FpCp+qZ_\infty^{p,q} = \{z \in F_pC_{p+q} : dz = 0\} = Z(F_pC)_{p+q} \cap F_pC_{p+q} B∞p,q=B(C)p+q∩FpCp+qB_\infty^{p,q} = B(C)_{p+q} \cap F_pC_{p+q}

Thus: E∞p,qβ‰…Z(FpC)p+qB(C)p+q∩FpCp+q+Z(Fpβˆ’1C)p+q=FpHp+q(C)/Fpβˆ’1Hp+q(C)E_\infty^{p,q} \cong \frac{Z(F_pC)_{p+q}}{B(C)_{p+q} \cap F_pC_{p+q} + Z(F_{p-1}C)_{p+q}} = F_pH_{p+q}(C) / F_{p-1}H_{p+q}(C)

This shows convergence to the associated graded of the filtered homology.

β– 
Remark

This construction is the prototype for all spectral sequences. The key insight is that successive pages approximate homology by allowing differentials to "cross" more filtration levels, eventually capturing the full differential structure.