TheoremBase

Smooth Diffeomorphism of Euclidean or Half-Space Domains, Jacobian, and Pullback

Statement

Let n∈n\in N\mathbb{N} and let Ω,Ω′\Omega,\Omega' be admissible domains in Euclidean space Rn\mathbb{R}^n in the sense of Continuous n-Form, Support, and Zero Extension on a Euclidean or Half-Space Domain.

A smooth diffeomorphism F:Ω→Ω′F:\Omega\to\Omega' is a bijection with the following two properties.

  1. For every x∈Ωx\in\Omega there exist open sets W,W′⊆RnW,W'\subseteq\mathbb{R}^n with x∈Wx\in W and F(x)∈W′F(x)\in W', and a smooth map G:W→W′G:W\to W' such that G(y)=F(y)G(y)=F(y) for every y∈W∩Ωy\in W\cap\Omega.
  2. The analogous local smooth extension condition holds for the inverse map F−1:Ω′→ΩF^{-1}:\Omega'\to\Omega at every point of Ω′\Omega'.

For x∈Ωx\in\Omega, the Jacobian matrix JF(x)J_F(x) of FF at xx is the Jacobian matrix DG(x)DG(x) of a local smooth extension GG of FF at xx as in property 1; by Jacobian Matrix of a Local Smooth Extension on an Admissible Domain this matrix exists and does not depend on the choice of GG.

We say that FF is orientation-preserving if for every x∈Ωx\in\Omega the determinant of JF(x)J_F(x) satisfies

det⁡JF(x)>0.\det J_F(x)>0.

Finally, let k∈N∪{0}k\in\mathbb{N}\cup\{0\}. For k≥1k\ge 1, let ω\omega be an assignment which to each point x′∈Ω′x'\in\Omega' assigns an alternating kk-linear form ωx′\omega_{x'} on Rn\mathbb{R}^n. The pullback F∗ωF^{*}\omega is the assignment which to each x∈Ωx\in\Omega assigns the alternating kk-linear form given by

(F∗ω)x(v1,…,vk)=ωF(x)(JF(x)v1,…,JF(x)vk)(F^{*}\omega)_x(v_1,\dots,v_k)=\omega_{F(x)}\bigl(J_F(x)v_1,\dots,J_F(x)v_k\bigr)

for all vectors v1,…,vk∈Rnv_1,\dots,v_k\in\mathbb{R}^n, where JF(x)vrJ_F(x)v_r is the matrix-vector product. For k=0k=0 we use the convention, consistent with Differential k-Form on an Open Subset of Euclidean Space, that an assignment of alternating 00-linear forms on Ω′\Omega' is a real-valued function ω:Ω′→R\omega:\Omega'\to\mathbb{R}, and the pullback is defined by

(F∗ω)(x)=ω(F(x))(F^{*}\omega)(x)=\omega(F(x))

for every x∈Ωx\in\Omega.

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