Composing smooth embeddings with diffeomorphisms #
Mathlib defines Manifold.IsSmoothEmbedding as an immersion that is also a topological embedding,
and lists Diffeomorph.isSmoothEmbedding as a TODO in
Mathlib/Geometry/Manifold/SmoothEmbedding.lean, because a general composite of immersions has to
combine two complements.
Composing with a diffeomorphism is elementary, and
TauCeti/Geometry/Manifold/Immersion.lean settles it at the level of immersions: a diffeomorphism
carries charts to charts, so both composites are immersions with the same complement. This file
adds the topological half — composing with a diffeomorphism on either side preserves being a
topological embedding — and packages the result as operations on the bundled type
TauCeti.SmoothEmbedding.
Since the composition is transparent, the bundled operations are just reindexing: a
TauCeti.SmoothEmbedding may be reparametrised by a diffeomorphism of its source
(TauCeti.SmoothEmbedding.compDiffeomorph) or transported by a diffeomorphism of its target
(TauCeti.SmoothEmbedding.transDiffeomorph). Reparametrisation leaves the image alone and
transport moves it by the ambient diffeomorphism, and the two assemble into an action of the
self-diffeomorphism group TauCeti.Diff of the target and a right action of that of the source;
the two actions commute, which is recorded as a SMulCommClass instance.
This is the transport layer that layer 4 of the geometric-topology roadmap
(TauCetiRoadmap/GeometricTopology/README.md) needs for its geometric presentations, where a knot
presentation is a smooth embedding of the circle into the ambient manifold N: reversing the
orientation of such a presentation and rotating its parametrisation are precisely precomposition
with a diffeomorphism of the circle, while the ambient Diff(N)-action is postcomposition.
Main results #
TauCeti.isSmoothEmbedding_comp_diffeomorphandTauCeti.isSmoothEmbedding_diffeomorph_comp, with their_iffcompanions: smooth embeddings are stable under composition with a diffeomorphism on either side.TauCeti.isSmoothEmbedding_comp_continuousLinearEquiv: a smooth embedding of a vector space stays one after a linear change of coordinates of the source, which changes its model.Diffeomorph.isSmoothEmbedding_comp: from a boundaryless source, a diffeomorphism onto a boundaryless manifold with a different finite-dimensional model transports smooth embeddings.TauCeti.isSmoothEmbedding_diffeomorph: a diffeomorphism is a smooth embedding.TauCeti.isSmoothEmbedding_continuousLinearEquiv: a continuous linear equivalence between normed spaces is a smooth embedding.
Main definitions #
TauCeti.SmoothEmbedding.ofDiffeomorph: a diffeomorphism as a bundled smooth embedding.TauCeti.SmoothEmbedding.ofContinuousLinearEquiv: a continuous linear equivalence as a bundled smooth embedding.TauCeti.SmoothEmbedding.compDiffeomorph: reparametrise a bundled smooth embedding by a diffeomorphism of its source.TauCeti.SmoothEmbedding.transDiffeomorph: transport a bundled smooth embedding by a diffeomorphism of its target.- the
MulAction (Diff J N n) (SmoothEmbedding I J n M N)given by transport, and the reparametrisation action of(Diff I M n)ᵐᵒᵖ.
References #
- M. Hirsch, Differential Topology, Springer GTM 33 (1976), Chapter 1, for immersions and embeddings and their behaviour under composition.
Reparametrising a smooth embedding by a diffeomorphism of the source gives a smooth embedding.
Precomposing a smooth embedding of a vector space with a continuous linear equivalence gives a smooth embedding, for the new source model.
Transporting a smooth embedding by a diffeomorphism of the target gives a smooth embedding.
Precomposition with a diffeomorphism of the source neither creates nor destroys a smooth embedding.
Postcomposition with a diffeomorphism of the target neither creates nor destroys a smooth embedding.
A diffeomorphism is a smooth embedding.
This is the statement Mathlib lists as the TODO Diffeomorph.isSmoothEmbedding in
Mathlib/Geometry/Manifold/SmoothEmbedding.lean; the name is flat here because a Diffeomorph
namespace nested in TauCeti would break dot notation on Mathlib's type.
A continuous linear equivalence between normed spaces is a smooth embedding.
Transporting a smooth embedding from a boundaryless manifold by a diffeomorphism of the target onto a boundaryless manifold gives a smooth embedding, also when the new target has a different finite-dimensional model.
A diffeomorphism, as a bundled smooth embedding.
Instances For
A continuous linear equivalence between normed spaces, as a bundled smooth embedding.
Instances For
A diffeomorphism is onto, so as a smooth embedding it has full image.
Not a simp lemma: coe_ofDiffeomorph already rewrites the left-hand side, so this form is never
in normal form.
Reparametrisation. A diffeomorphism e : M' ≃ₘ M of the source turns a bundled smooth
embedding f : M → N into the bundled smooth embedding f ∘ e : M' → N. For a geometric knot
presentation, an embedding of the circle into the ambient manifold N, this is the change of
parametrisation of the knot, orientation reversal included.
Instances For
Reparametrising by the identity diffeomorphism changes nothing.
Reparametrising twice is reparametrising by the composite diffeomorphism.
Reparametrisation does not move the image of an embedding.
Not a simp lemma: coe_compDiffeomorph already rewrites the left-hand side, so this form is never
in normal form.
Ambient transport. A diffeomorphism e : N ≃ₘ P of the target turns a bundled smooth
embedding f : M → N into the bundled smooth embedding e ∘ f : M → P. For a geometric knot
presentation this is the action of the ambient diffeomorphism group on knots.
Instances For
Transporting by the identity diffeomorphism changes nothing.
Transporting twice is transporting by the composite diffeomorphism.
Ambient transport moves the image of an embedding by the ambient diffeomorphism.
Not a simp lemma: coe_transDiffeomorph already rewrites the left-hand side, so this form is
never in normal form.
Reparametrising and transporting commute: they act on opposite sides of the embedding. This is the normal form for a mixed reparametrisation-and-transport, with the transport innermost.
The self-diffeomorphism group of the target acts on the smooth embeddings into it, by ambient
transport. This is the action of Diff(N) on geometric knot presentations in N.
The self-diffeomorphism group of the source acts on the smooth embeddings out of it on the right, by reparametrisation; the action is recorded on the opposite group.
Ambient transport and reparametrisation commute, so the two actions are compatible.