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Local side-preserving extensions of half-space transitions
Statement
Let and be a smooth diffeomorphism between relatively open subsets of . At every there are Euclidean open neighborhoods of and of and a smooth diffeomorphism extending locally, such that maps the positive, zero, and negative sides of onto the corresponding sides in .
Facts & Assumptions
Smooth invariance of the manifold boundary: A smooth diffeomorphism between relatively open half-space sets carries face points to face points and relative-interior points to relative-interior points; consequently and are intrinsic.
Chain rule for smooth half-space maps: If and are smooth maps between relatively open half-space sets, then is smooth and .
Half-space extensions agreeing on a relatively open set have the same derivatives there: If two smooth Euclidean extensions agree on a relatively open subset of , then all of their derivatives agree at every point of that subset.
The Euclidean inverse function theorem: Let , let be open, let be , and let . If is invertible, then there are open sets with and such that is bijective. Its inverse is , and Thus is a local diffeomorphism at .
Newton–Leibniz needs only continuity on , differentiability on , and a Riemann-integrable extension of the interior derivative: Let . Suppose is continuous on and differentiable on . If is Riemann integrable and then No derivative of at either endpoint is assumed, and the two endpoint values assigned to the integrable extension do not enter the conclusion.
Proof
Given: The objects and hypotheses in the statement above.
The face maps into the face and the interior into the interior. The half-space chain rule applied to shows that is invertible. Derivatives do not depend on the smooth extensions chosen near .
Write the last component of a local extension as . On a small face disk , so its tangential derivatives vanish. Since for small , . Invertibility and the zero tangential entries in the last row exclude zero, so .
After shrinking to a product neighborhood, continuity makes positive there. The one-variable fundamental theorem gives with , also for negative . Hence has exactly the sign of .
Apply the Euclidean inverse theorem to the extension and shrink its inverse neighborhoods inside that product neighborhood. The inverse is smooth: its derivative is the inverse derivative matrix composed with the inverse map, and repeated differentiation bootstraps the stated inverse to every finite order. The sign identity gives both inclusions of each side equality. For the tangential row is empty and the same positive derivative argument applies.
Depends on
- Smooth invariance of the manifold boundary
- Chain rule for smooth half-space maps
- Half-space extensions agreeing on a relatively open set have the same derivatives there
- The Euclidean inverse function theorem
- Newton–Leibniz needs only continuity on $[a,b]$, differentiability on $(a,b)$, and a Riemann-integrable extension of the interior derivative
Used by
Dependency tree · two levels
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Sources
- Lee Proposition 16.3, p.404, together with the exact published boundary chain rule and inverse theorem (standard reference, not scraped)