Maps between intrinsic normal spaces #
A commuting square of differentiable maps between two smooth embeddings induces a continuous linear map between their normal spaces. The ambient differential carries tangent vectors to tangent vectors, so it descends to the quotient. The induced map is an equivalence when the base differential is surjective and the ambient map is a local diffeomorphism. In particular, these equivalences identify intrinsic normal fibres in different submanifold coordinates, without choosing a metric or a complement.
The square need only commute near the chosen base point. This local formulation allows the construction to be applied to coordinate maps and to their restrictions.
A globally commuting square of differentiable maps also induces a map of intrinsic normal total spaces, acting on each fibre by the normal map; these maps respect identities and composition.
References #
- M. Hirsch, Differential Topology, Springer GTM 33 (1976), Chapter 4, ยงยง5โ6, for the normal-bundle and tubular-neighbourhood setting.
- Tau Ceti's
ContinuousLinearEquiv.quotientEquivtopologically bundles Mathlib'sSubmodule.Quotient.equiv.
The ambient differential in a locally commuting square carries tangent vectors to tangent vectors.
The normal map induced by a square commuting near x. On an ambient tangent class,
it is given by the differential of v.
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The induced normal map commutes with the continuous quotient maps.
The induced normal map sends the class of w to the class of dv(w).
The identity square induces the identity on the normal space.
A differentiable commuting square induces a map of intrinsic normal total spaces. On each fibre it is the normal map induced by the ambient differential.
Instances For
The induced total map applies the normal map in each fibre.
The identity square induces the identity of the normal bundle.
Composing locally commuting squares composes their induced normal maps. The composite square is obtained from the two given germs.
Composing commuting squares composes their induced maps on normal total spaces.
If the base differential is surjective, the ambient differential carries the tangent range onto the target tangent range.
A commuting square identifies the intrinsic normal fibres when the base differential is surjective and the ambient map is a local diffeomorphism at the chosen point.
Instances For
The normal-fibre equivalence has the normal map as its underlying continuous linear map.
On representatives, the normal-fibre equivalence applies the ambient differential.
On representatives, the inverse normal-fibre equivalence applies the inverse ambient differential, after identifying the equal ambient base points.