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) be a chain complex with an increasing filtration:
β―βFpβ1βCβFpβCβFp+1βCββ―βC
such that d(FpβC)βFpβC. Then there exists a spectral sequence with:
E0p,qβ=FpβCp+qβ/Fpβ1βCp+qβE1p,qβ=Hp+qβ(FpβC/Fpβ1βC)
converging to Hββ(C) with filtration induced from C.
Proof
Step 1: Define E0β and d0β. Set:
E0p,qβ=FpβCp+qβ/Fpβ1βCp+qβ
The differential d on C induces d0β on E0β by passing to quotients. Since d(FpβC)βFpβC, this is well-defined.
Step 2: Compute E1β. By definition:
E1p,qβ=Hp+qβ(E0p,ββ,d0β)=Hp+qβ(FpβC/Fpβ1βC)
This is the homology of the p-th graded piece.
Step 3: Construct d1β. The differential d1β:E1p,qββE1pβ1,qβ comes from the connecting homomorphism in the long exact sequence:
β―βHnβ(Fpβ1βC)βHnβ(FpβC)βHnβ(FpβC/Fpβ1βC)ββHnβ1β(Fpβ1βC)ββ―
Specifically, d1β=β:E1p,qββE1pβ1,qβ where we identify:
E1p,qβ=Hp+qβ(FpβC/Fpβ1βC)
Step 4: Verify d1ββd1β=0. This follows from the exactness of the long exact sequence: the composition of consecutive connecting homomorphisms is zero.
Step 5: Compute E2β. By definition:
E2p,qβ=ker(d1β:E1p,qββE1pβ1,qβ)/Im(d1β:E1p+1,qββE1p,qβ)
From the long exact sequence, this is related to the relative homology groups.
Step 6: Higher pages. For rβ₯2, define:
Zrp,qβ={zβFpβCp+qβ:dzβFpβrβCp+qβ1β}Brp,qβ={bβFpβCp+qβ:b=dz+bβ²Β whereΒ zβFp+rβ1βCp+q+1β,bβ²βFpβ1βCp+qβ}
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.