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If I know that $\Phi_\varepsilon$ is bounded in $L^{\infty}(\mathbb{R}^{2d})$ and that $\nabla \Phi_\varepsilon$ is bounded in $L^{\infty}(\mathbb{R}^{2d})$, is it true that $\nabla \Phi_\varepsilon \to \nabla \Phi$, where $\Phi$ is the limit of $\Phi_\varepsilon$ as $\varepsilon \to 0$ (in the weak sense)?

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$\newcommand\ep\varepsilon$No. E.g., let $\Phi:=0$ and $$\Phi_\ep(x)=\ep e^{-|x|^2}\cos\frac{|x|}\ep$$ for all $x$, where $|x|$ is the Euclidean norm of $x$.

Then $\Phi_\ep\to\Phi$ but $\nabla\Phi_\ep\not\to\nabla\Phi$.

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  • $\begingroup$ We also know that both two sequences (functions and gradients) are uniformly bounded. $\endgroup$
    – Markus
    Commented Apr 5, 2022 at 7:17

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