The index filtration of a twisted complex #
The index filtration F_p consists of functions supported on generators of index at most p.
For a finite generator set it is bounded below, exhaustive, and increasing. One-sidedness of the
cocycle ensures that the twisted differential preserves it: the coefficient differential keeps
the generator index, and every twisting term strictly lowers it.
Intersecting F_p with the total grading gives an actual cochain complex, with inclusion maps
between its stages. The quotient F_p / F_{p-1} is identified with the copies of the coefficient
module indexed by generators of index exactly p. Under this identification its induced
differential acts coordinatewise by the coefficient differential. These are the filtered-complex
and associated-graded inputs to the spectral sequence of a twisted complex. Restriction to the
generators of a fixed index gives a short exact sequence of the filtration stages and the
zero-cocycle complex, with the restriction map as cokernel of the stage inclusion.
References #
- J.-F. Barraud, M. Damian, V. Humilière, A. Oancea, Floer homology with DG coefficients. Applications to cotangent bundles, arXiv:2404.07953, Definition 3.38.
- J.-F. Barraud, M. Damian, V. Humilière, A. Oancea, Morse homology with differential graded coefficients, Progress in Mathematics 360, Birkhäuser, 2025, Chapters 3--4.
The index filtration is increasing.
Above every generator index the filtration stage is all of P → M.
Below every generator index the filtration vanishes.
A finite index filtration is bounded, including when there are no generators.
Every element lies in some stage of a finite index filtration.
A family extended by zero from the generators of index p lies in the p-th piece of the
index filtration.
A family of total degree n on the generators of index p, extended by zero, has total
degree n.
Every twisting contribution from F_p vanishes at a generator of index at least p.
One-sidedness makes the twisted differential preserve every index-filtration stage.
At index p, the differential of an element of F_p is its coefficient differential.
Every twisting contribution has strictly smaller generator index.
Restriction to generators of index p intertwines the differential on F_p with the
coefficient differential, which is the twisted differential of the zero cocycle.
The preceding filtration stage as a submodule of F_p.
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The kernel of projection to index p is exactly F_{p-1} inside F_p.
Every family on the generators of index p is the projection of an element of F_p.
The associated graded module F_p / F_{p-1} is the family of coefficient modules on
exactly the generators of index p.
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The restriction of the twisted differential to F_p.
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The differential induced on F_p / F_{p-1} by the twisted differential.
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The quotient differential sends the class of f to the class of its twisted differential.
Under the associated graded equivalence, the induced differential is the coefficient
differential on each generator of index exactly p.
The induced associated graded differential squares to zero whenever the coefficient differential does.
The twisted differential carries the degree-n part of F_p into degree n+1 in F_p.
The pth stage of the index filtration as a cochain complex of modules.
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The degree terms of the filtered complex are the intersections of filtration and grading.
Identify a term of the filtered complex with the intersection of index and total degree.
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The filtered cochain differential acts by the ambient twisted differential.
The canonical cochain inclusion of a filtration stage into a later stage.
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The transition map is inclusion on the underlying homogeneous functions.
The inclusion F_p ⟶ F_q is injective in each degree.
The canonical inclusion of an index-filtration stage into the twisted complex.
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The stage inclusion preserves the underlying homogeneous function.
Each filtration stage embeds as a subcomplex of the twisted complex.
Transition maps commute with the inclusions into the twisted complex.
Transition maps commute with the inclusions into the twisted complex.
The transition map from a filtration stage to itself is the identity.
The transition maps of the index filtration compose.
The transition maps of the index filtration compose.
The index filtration as actual subobjects of the twisted cochain complex.
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The filtration subobject is represented by the canonical stage inclusion.
In degree n, the p-th filtration stage consists of the homogeneous families vanishing
on generators of index greater than p.
The subobjects of the twisted complex form an increasing filtration.
Above all generator indices the subobject filtration is the whole twisted complex.
Below all generator indices the subobject filtration vanishes.
The actual subcomplex filtration is bounded below and exhaustive for finite generators.
The graded pieces of the index filtration #
The restriction F_p ⟶ ℳ ⊗ ⟨P_p⟩ from the pth step of the index filtration to the twisted
complex of the zero cocycle on the generators P_p of index p.
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The short complex F_p ⟶ F_{p+1} ⟶ ℳ ⊗ ⟨P_{p+1}⟩ of the index filtration, whose second map
restricts to the generators of index p + 1.
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The first map in twistedIndexShortComplex is the inclusion F_p ⟶ F_{p+1}.
The second map in twistedIndexShortComplex is restriction to the generators of index
p + 1.
The restriction F_p ⟶ ℳ ⊗ ⟨P_p⟩ is surjective in each degree: a function on the generators
of index p extends by zero to a function supported on the generators of index at most p.
An element of F_{p+1} whose restriction to the generators of index p + 1 vanishes comes
from F_p.
The graded pieces of the index filtration. The sequence
F_p ⟶ F_{p+1} ⟶ ℳ ⊗ ⟨P_{p+1}⟩ is short exact: the quotient F_{p+1} / F_p is the twisted complex
of the zero cocycle on the generators of index p + 1.
Restriction to the generators of index p + 1 is a cokernel of the inclusion
F_p ⟶ F_{p+1}.