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.