Whitney’s extension

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  • In this paper, we solve the following extension problem. Problem 1. Suppose we are given a function f : E → R, where E is a given subset of Rn. How can we decide whether f extends to a Cm−1,1 function F on Rn ? Here, m ≥ 1 is given. As usual, Cm−1,1 denotes the space of functions whose (m − 1)rst derivatives are Lipschitz 1. We make no assumption on the set E or the function f. This problem, with Cm in place of Cm−1,1, goes back to Whitney [15], [16], [17]. To answer it, we prove the following sharp form of the Whitney extension theorem....

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  • Let f be a real-valued function on a compact set in Rn , and let m be a positive integer. We show how to decide whether f extends to a Cm function on Rn . Introduction Continuing from [F2], we answer the following question (“Whitney’s extension problem”; see [hW2]). Question 1. Let ϕ be a real-valued function defined on a compact subset E of Rn . How can we tell whether there exists F ∈ C m (Rn ) with F = ϕ on E? Here, m ≥ 1 is given, and C m (Rn ) denotes the space...

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  • Characteristic cohomology classes, defined in modulo 2 coefficients by Stiefel [26] and Whitney [28] and with integral coefficients by Pontrjagin [24], make up the primary source of first-order invariants of smooth manifolds. When their utility was first recognized, it became an obvious goal to study the ways in which they admitted extensions to other categories, such as the categories of topological or PL manifolds; perhaps a clean description of characteristic classes for simplicial complexes could even give useful computational techniques.

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