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An exact couple generates a spectral sequence
Statement
An initial homological exact couple gives, by repeated derivation, a homological spectral sequence starting at with . Write for consecutive shifted maps, including . Define subobjects of the original by Then canonically . Under this identification, if locally , then . An arbitrary exact couple is not asserted to have an abutment.
Facts & Assumptions
Exact couple supplies , , , with the initial of degree zero.
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.
Homological spectral sequence requires square-zero differentials and specified homology-to-next-page isomorphisms.
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].
Applying the derived-couple theorem to any page- couple produces a page- couple whose object is exactly the homology of the preceding differential. The associated differentials square to zero and their degrees are . 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 .
The kernels of successive powers increase and their images decrease. Since , for every . Thus and each stated quotient exists. For , and .
For through , take a local at with . The arrow lies in every since . Two such lifts differ by and hence give the same class modulo in the target. Replacing by a local representative changes by zero. Consequently the rule defines a unique map on , by quotient descent. Its degree is and its square is zero: for the representative its image is zero, so the next lift may be taken to be zero. At this rule is the original .
The kernel of this map is represented by exactly . Indeed a zero image means locally with . Then , so locally and . Conversely if , one may take and then . Both containments descend. The incoming image is exactly : every output has , and conversely if , then has a local lift with . This belongs to the appropriate and its image is . Thus the homology of the quotient at page is canonically .
To match these quotient pages with repeated derived couples in step 1.1, note that at the th couple the object is inside the original . Its map is induced by the original on , and its map sends to . These assertions hold initially. On deriving once, the image of the restricted is ; the new is induced by the same original on the new cycles in step 3.1; and taking one more -preimage changes into , so the new sends to . 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.
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 , and canonical kernels, images and quotients, requires neither infinite sums nor AC. No target filtration or abutment has been constructed or inferred.
Depends on
Used by
Dependency tree · two levels
7 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Stacks Project, Definition 12.21.3 and Lemma 12.21.4 (full local proof supplied) (standard reference, not scraped)