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TheoremStatement: AI-adaptedProof: AI-adaptedPipeline-generatedjudge pass (gpt-5.6-terra)audited 2026-09-12
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An exact couple generates a spectral sequence

Statement

An initial homological exact couple (D,E,i,j,k) gives, by repeated derivation, a homological spectral sequence starting at E1=E with deg⁡dr=(−r,r−1). Write im for m consecutive shifted i maps, including i0=1. Define subobjects of the original Ep,q by Np,qr=kp,q−1(im⁡(ir−1:Dp−r,q+r−1→Dp−1,q)), Bp,qr=jp,q(ker⁡(ir−1:Dp,q→Dp+r−1,q−r+1)). Then canonically Ep,qr≅Np,qr/Bp,qr. Under this identification, if locally ke=ir−1x, then dr[e]=[jx]. An arbitrary exact couple is not asserted to have an abutment.

Facts & Assumptions

[F1]

Exact couple supplies ker⁡i=im⁡k, ker⁡k=im⁡j, ker⁡j=im⁡i, with the initial j of degree zero.

[F2]

Derived exact couple defines the derived image and homology objects and their maps; The derived couple is exact allows their repeated derivation and gives the new degrees.

[F3]

Homological spectral sequence requires square-zero differentials and specified homology-to-next-page isomorphisms.

[F4]

Spectral sequence subquotient and local lifting calculus supplies finite epic lifts, natural quotient identifications and descent of containments.

Proof

Given: The initial exact couple and the indexed subobjects in the statement. Every local lift is after a finite epic pullback, with the descent meaning of [F4].

1.1F2F3given

Applying the derived-couple theorem to any page-r couple produces a page-(r+1) couple whose E object is exactly the homology of the preceding jk differential. The associated differentials square to zero and their degrees are (−r,r−1). Starting with the given initial couple and repeating this construction for each positive integer therefore supplies the objects, differentials and homology identifications required for a spectral sequence. The initial page is E1=E.

1.2F1F4given

The kernels of successive i powers increase and their images decrease. Since kj=0, Br⊆Ns for every r,s≥1. Thus Br⊆Br+1⊆Nr+1⊆Nr and each stated quotient exists. For r=1, N1=E and B1=j(ker⁡1)=0.

2.1F1F4step 1.2

For e through Np,qr, take a local x at Dp−r,q+r−1 with ke=ir−1x. The arrow jx lies in every Ns since kjx=0. Two such lifts differ by ker⁡ir−1 and hence give the same class modulo Br in the target. Replacing e by a local Br representative jz changes ke by zero. Consequently the rule [e]↦[jx] defines a unique map on Nr/Br, by quotient descent. Its degree is (−r,r−1) and its square is zero: for the representative jx its k image is zero, so the next lift may be taken to be zero. At r=1 this rule is the original jk.

3.1F1F4step 2.1

The kernel of this map is represented by exactly Nr+1. Indeed a zero image means locally jx=jz with ir−1z=0. Then x−z∈ker⁡j=im⁡i, so locally x−z=iy and ke=ir−1x=iry. Conversely if ke=iry, one may take x=iy and then jx=0. Both containments descend. The incoming image is exactly Br+1/Br: every output jx has irx=i(ke)=0, and conversely if irx=0, then ir−1x∈ker⁡i=im⁡k has a local lift e with ke=ir−1x. This e belongs to the appropriate Nr and its image is jx. Thus the homology of the quotient at page r is canonically Nr+1/Br+1.

4.1F2F4step 1.1step 3.1

To match these quotient pages with repeated derived couples in step 1.1, note that at the rth couple the D object is im⁡ir−1 inside the original D. Its k map is induced by the original k on Nr, and its j map sends ir−1x to [jx]. These assertions hold initially. On deriving once, the image of the restricted i is im⁡ir; the new k is induced by the same original k on the new cycles in step 3.1; and taking one more i-preimage changes iry into ir−1y, so the new j sends iry to [jy]. Step 3.1 identifies the new homology quotient and its transition by the inclusion of its numerator. This proves the asserted compatibility at every stage of the iteration.

5.1F3step 1.2step 4.1∎

Hence the stated subquotients and differentials describe precisely the spectral sequence of the exact couple, with specified canonical transition isomorphisms. The zero couple gives zero quotients at all pages. The use of finite composites at each fixed r, and canonical kernels, images and quotients, requires neither infinite sums nor AC. No target filtration or abutment has been constructed or inferred.

Depends on

Dependency tree · 2 levels

Sources