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
