EN
In Example 1, we describe a subset X of the plane and a function on X which has a $𝐶^k$-extension to the whole $ℝ^2$ for each 𝑘 finite, but has no $𝐶^∞$-extension to $ℝ^2$. In Example 2, we construct a similar example of a subanalytic subset of $ℝ^5$; much more sophisticated than the first one. The dimensions given here are smallest possible.