\( \definecolor{colordef}{RGB}{249,49,84} \definecolor{colorprop}{RGB}{18,102,241} \)
For each \(k \in \mathbb{N}^*\), find a function \(\mathbb{R} \to \mathbb{R}\) that is \(C^{k-1}\) but not \(C^k\). (Hint: generalize the previous exercise.)
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