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aglearner
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Applying min-max to find a critical point in a ball

Let $\mathbb B^n$ be a unit ball in $\mathbb R^n$ and let $F$ be a smooth function on it. Let $\mathbb B^k$ be a unit ball in $\mathbb R^k$.

Question. Is it true that $F$ has a critical point in the interior of $\mathbb B^n$ if there exists a smooth surjective map $\varphi:\mathbb B^n\to \mathbb B^k$, that has the following properties:

  1. For any point $x\in \mathbb B^k$ the minimum of $F|_{\varphi^{-1}(x)}$ is attained in the interior of $\mathbb B^n$.

  2. The maximum $$\max_{y\in B}\min_{x\in \varphi^{-1}(y)} F(x)$$ is attained in the interior of $\mathbb B^k$.

Comment. The answer is clearly positive if the differential of $\varphi$ is surjective on the whole ball $\mathbb B^n$.

I would be grateful for a reference (or a counter-example which sounds less likely...).

aglearner
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