Compactly supported de Rham cohomology #
Smooth real-valued forms with compact topological support form a submodule of all smooth forms.
The exterior derivative does not enlarge supports, so these forms are a subcomplex of the de Rham
complex, the compactly supported de Rham complex; its cohomology is compactly supported de Rham
cohomology H^•_c.
A compactly supported closed form has a class in H^•_c; every class arises this way, and a class
vanishes exactly when its representative is the exterior derivative of a compactly supported form.
The inclusion of complexes induces the comparison H^•_c(M) → H^•_dR(M), which takes the de Rham
class of the representative; it is an isomorphism on compact manifolds, where every form is
compactly supported.
On a manifold modelled on a finite-dimensional space, every alternating form of degree above the
dimension is zero, so H^k_c(M) vanishes for k > dim M.
Pullback along a smooth proper map preserves compact support, so H^•_c is contravariant for
smooth proper maps, and a diffeomorphism induces an isomorphism. Pullback along a general smooth
map does not preserve compact support: the constant map ℝ → pt pulls the constant function 1
back to a function without compact support.
Main definitions #
TauCeti.compactlySupportedForms I M k: compactly supported smoothk-forms.TauCeti.compactlySupportedClosedForms I M k: compactly supported closed smoothk-forms.TauCeti.compactlySupportedDeRhamComplex I M: the compactly supported de Rham complex.TauCeti.compactlySupportedDeRhamCohomology I M k: its cohomology.TauCeti.compactlySupportedDeRhamCohomology.mk: the class of a compactly supported closed form.TauCeti.compactlySupportedDeRhamCohomology.toDeRhamCohomology: the comparison with de Rham cohomology.TauCeti.compactlySupportedDeRhamComplex.ι: the inclusion into the de Rham complex.TauCeti.compactlySupportedDeRhamComplex.map: the cochain map induced by a smooth proper map.TauCeti.compactlySupportedDeRhamCohomology.map: the induced map on cohomology.TauCeti.compactlySupportedDeRhamCohomology.equivOfDiffeomorph: the induced isomorphism for a diffeomorphism.TauCeti.subsingleton_compactlySupportedDeRhamCohomology_of_finrank_lt:H^k_c(M) = 0above the dimension.
References #
- John M. Lee, Introduction to Smooth Manifolds, 2nd ed., Chapter 17, "Cohomology with Compact Support".
The submodule of smooth real-valued k-forms with compact topological support.
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A smooth form is compactly supported when its topological support is compact.
Transporting the degree of a smooth form preserves compact support.
On a compact manifold every smooth form is compactly supported.
The exterior derivative of a compactly supported smooth form is compactly supported.
The exterior derivative of compactly supported smooth forms, which does not enlarge supports.
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The exterior derivative of a compactly supported form is its exterior derivative as a smooth form.
Successive exterior derivatives of compactly supported forms compose to zero.
The compactly supported de Rham complex: compactly supported smooth real-valued forms with the exterior derivative.
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The canonical identification of the degree-k object with compactly supported smooth
k-forms.
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Under the canonical identifications of the objects, the differential of the compactly supported de Rham complex is the exterior derivative.
Under the canonical identifications of the objects, the differential of the compactly supported de Rham complex is the exterior derivative.
The differential of the compactly supported de Rham complex, evaluated on an element.
A closed compactly supported form, viewed in the compactly supported de Rham complex, is killed by the differential.
Compactly supported de Rham cohomology is the categorical homology of the compactly supported de Rham complex.
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The inclusion of the compactly supported de Rham complex into the de Rham complex.
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Under the canonical identifications of the objects, the inclusion is the inclusion of compactly supported forms into smooth forms.
Under the canonical identifications of the objects, the inclusion is the inclusion of compactly supported forms into smooth forms.
On a compact manifold the compactly supported de Rham complex is the de Rham complex.
The submodule of compactly supported closed smooth forms.
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A smooth form is compactly supported and closed when it is both compactly supported and closed.
A compactly supported closed form is compactly supported.
A compactly supported closed form is closed.
In degree at least the model dimension, every compactly supported form is closed.
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Viewing a top-degree compactly supported form as closed does not change the smooth form.
Compactly supported exact forms are exterior derivatives of compactly supported forms in positive degree, and zero in degree zero.
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There are no nonzero compactly supported exact forms in degree zero.
In positive degree, compactly supported exact forms are the image of compactly supported forms under the exterior derivative.
A compactly supported exact form is exact.
A compactly supported exact form is compactly supported and closed.
The compactly supported cohomology class of a compactly supported closed form.
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The class map is the homology class map of the inclusion of compactly supported closed forms into the compactly supported de Rham complex.
Every compactly supported cohomology class is represented by a compactly supported closed form.
A compactly supported closed form represents zero precisely when it is the exterior derivative of a compactly supported form.
The kernel of the class map is the submodule of compactly supported exact forms inside compactly supported closed forms.
Two compactly supported closed forms have the same class precisely when they differ by a compactly supported exact form.
The comparison map from compactly supported to ordinary de Rham cohomology, induced on cohomology by the inclusion of complexes.
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The comparison map sends the class of a compactly supported closed form to the de Rham class of the underlying closed form.
On a compact manifold compactly supported and ordinary de Rham cohomology agree.
The map induced on cohomology by the inclusion of complexes is the comparison map.
Compactly supported cohomology vanishes in degrees above the dimension of the model, where every alternating form is zero.
Pullback of compactly supported smooth forms along a smooth proper map.
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The pullback of a compactly supported form is its pullback as a smooth form.
Pullback of compactly supported forms by the identity is the identity linear map.
Pullback of compactly supported forms reverses composition.
Pullback of compactly supported forms along a smooth proper map commutes with the exterior derivative.
Pullback along a smooth proper map preserves compactly supported exact forms.
Pullback of compactly supported closed forms along a smooth proper map.
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The pullback of a compactly supported closed form is its pullback as a smooth form.
The underlying rough form of the pullback of a compactly supported closed form is the rough pullback of its underlying rough form.
Pullback of compactly supported closed forms by the identity is the identity linear map.
Pullback of compactly supported closed forms reverses composition.
The contravariant map on compactly supported de Rham cohomology induced by a smooth proper map.
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The induced map sends the class of a compactly supported closed form to the class of its pullback.
The map on compactly supported cohomology induced by the identity is the identity.
Pullback on compactly supported cohomology reverses composition of smooth proper maps.
The comparison with de Rham cohomology commutes with pullback along smooth proper maps.
A diffeomorphism Φ : M ≃ M', which is a proper map, induces a linear equivalence
H^k_c(M') ≃ H^k_c(M) on compactly supported de Rham cohomology: pullback along Φ, with inverse
pullback along Φ.symm.
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The equivalence induced by a diffeomorphism is pullback along it.
The inverse of the equivalence induced by a diffeomorphism is pullback along its inverse.
The equivalence induced by a diffeomorphism sends the class of a compactly supported closed form to the class of its pullback.
The induced cochain maps compare complexes in one universe, so the model spaces share a universe, as do the manifolds.
The cochain map between compactly supported de Rham complexes induced by a smooth proper map.
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Under the canonical identifications of the objects, the induced cochain map is pullback of compactly supported forms.
Under the canonical identifications of the objects, the induced cochain map is pullback of compactly supported forms.
The cochain map induced by the identity is the identity.
The induced cochain maps reverse composition of smooth proper maps.
The map induced on cohomology by the cochain map of a smooth proper map is pullback on compactly supported cohomology.