Smooth pullback of differential forms #
A C^(n+1) map pulls a C^n fixed-coefficient differential form back to a C^n form.
The relative theorem uses derivatives within the source set, so it applies at boundary and
corner points. No invertibility, finite dimensionality, or completeness is required.
For smooth maps, SmoothForm.mpullback bundles this operation as a linear map on smooth forms.
Its underlying rough form is the existing TauCeti.mpullback. Pullback along a smooth map with a
smooth right inverse, such as a vector bundle projection, is injective. The diffeomorphisms of M
preserving a smooth form form a subgroup of the diffeomorphism group, its stabilizer
(TauCeti.SmoothForm.diffeomorphStabilizer).
Conversely, regularity descends along a local section: if s is a C^n local section of f
through x, a form is C^m near f x exactly when its pullback along f is C^m near x,
for m + 1 ≤ n (TauCeti.contMDiffAt_mpullback_form_iff_of_section).
In tangent coordinates, pullback precomposes the form's coordinates with those of Mathlib's
mfderivWithin, via ContinuousAlternatingMap.compContinuousLinearMap. This expresses
pullback regularity in terms of maps between fixed model fibres. For bundle-valued forms,
the file provides the coordinate identity inCoordinates_bundleMpullbackWithin.
References #
- John M. Lee, Introduction to Smooth Manifolds, 2nd ed., Chapter 14.
Read with the coefficient trivialization of V' at f x and the tangent trivializations at
x and f x, a bundle-valued pullback is the form's coordinates precomposed with the
coordinates of mfderivWithin. The target tangent trivialization must be valid at f y.
In fibre coordinates, pullback precomposes the form's coordinates with the derivative's
coordinates. The target tangent trivialization must be valid at f y.
A form of regularity C^m pulls back to a C^m section along a C^n map when m + 1 ≤ n.
The derivative defining pullback is taken within the unique-differentiability set s.
Pullback within a unique-differentiability set preserves C^m sections when the map is
C^n and m + 1 ≤ n.
Pointwise regularity of the pullback of a form.
Let f be C^n at x, m + 1 ≤ n, and let s be a C^n local section of f through x
over an open neighbourhood U of f x. Then a form is a C^m section near f x exactly when its
pullback along f is a C^m section near x: near f x, the form is the pullback of its
pullback along s. Local inverses of local diffeomorphisms and local sections of submersions are
such sections.
Pullback of a fixed-coefficient C^m form along a C^n map is C^m for m + 1 ≤ n.
In particular, smooth maps pull smooth forms back to smooth forms.
Pullback of a fixed-coefficient C^m form along a C^(m+1) map is C^m.
This is the form-level interface to the general bundle-section pullback theorem.
The pullback of a continuous form along a C¹ map u from a normed space, evaluated on fixed
vectors, is continuous. For u a map of the plane this is the integrand of a rectangle
integral.
For a continuous two-form φ and a C¹ map u from a normed space, the form evaluated on
the derivatives of u along two fixed vectors, z ↦ φ (u z) (du_z a, du_z b), is continuous.
For a continuous two-form φ and a map u from a normed space which is C¹ on an open set
U, the form evaluated on the derivatives of u along two fixed vectors,
z ↦ φ (u z) (du_z a, du_z b), is continuous on U.
Pulling a form back along a diffeomorphism and then along its inverse returns the form.
Pulling back along the retraction 0 ∘ π of a vector bundle onto its zero section factors
through the restriction to the zero section.
Pullback along a smooth map, as a linear map on smooth fixed-coefficient forms.
Instances For
The underlying rough form of smooth pullback is rough pullback.
Pullback of smooth forms commutes with transport of the degree.
Evaluation of smooth pullback on tangent vectors. Simplification also obtains this from
coe_mpullback and the rough pullback's application law.
Pullback of smooth forms along a surjective smooth map whose derivatives are all surjective is injective.
A pullback equation mpullback f hf φ' = c • φ of smooth forms, read at the point x as an
equation of the underlying rough forms.
If the pullback of φ' is c times φ at x, then the pullback of -φ' is -c times φ
at x.
Evaluation of the smooth pullback of a two-form on a pair of tangent vectors.
Evaluation of the pullback along the first projection of a product reads the first components of the tangent vectors.
Evaluation of the pullback along the second projection of a product reads the second components of the tangent vectors.
The pullback of a two-form along the first projection of a product, evaluated on two tangent vectors, reads their first components.
The pullback of a two-form along the second projection of a product, evaluated on two tangent vectors, reads their second components.
Pullback along a graph. Along the graph y ↦ (y, f y) of a map f differentiable at
x, the form pr₁^*α - pr₂^*β on M × M' pulls back at x to α - f^*β.
Pullback by the identity is the identity linear map.
Pullback reverses composition of smooth maps.
Pulling a form back along g and then along f is pulling it back along g ∘ f.
Pullback along a smooth map with a smooth right inverse is injective.
Pullback along the projection of a smooth vector bundle is injective: the zero section is a smooth right inverse of the projection.
If a diffeomorphism φ pulls β back to α, then its inverse pulls α back to β.
Pulling a form back along the inverse of a diffeomorphism φ and then along φ gives the form
back.
Pulling a form back along a diffeomorphism φ and then along its inverse gives the form back.
Pullback along a composite of diffeomorphisms is the composite of the pullbacks.
Pullback along a diffeomorphism φ transports pointwise proportionality: at x, the
pullback of α is c times the pullback of β exactly when α is c times β at φ x.
The stabilizer of a smooth form σ in the diffeomorphism group of M: the smooth
diffeomorphisms φ with φ^* σ = σ. For a symplectic form ω this is the group of
symplectomorphisms Symp(M, ω), and for a volume form the group of volume-preserving
diffeomorphisms.
Instances For
A diffeomorphism lies in the stabilizer of σ exactly when it pulls σ back to itself.
A diffeomorphism preserves a smooth form exactly when it preserves its value at every point:
(φ^* σ)_x = σ_x for all x.
A smooth form preserved by a diffeomorphism φ is preserved by every integer power of φ.
Smooth pullback is a homomorphism for every fixed continuous coefficient pairing.
Smooth real-valued pullback is a homomorphism for the usual wedge.