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Given a fixed value $\lambda>0$ let $u\in \dot{H}^1(\mathbb{R}^n)$ be a weak solution (or eigenfunction with eigenvalue $\lambda$) in dimension $n\geq 3$ to the following PDE, $$-\Delta u = \lambda \rho u$$ where $\rho\in L^{n/2}(\mathbb{R}^n)\cap L^{\infty}(\mathbb{R}^n)$ and $\lambda >0.$ Can we conclude that $u\in L^{\infty}(\mathbb{R}^n)?$

I know by standard elliptic regularity theory we can deduce that $u\in C^{\alpha}_{\operatorname{loc}}(\mathbb{R}^n)$, but does this imply that the solution is in $L^{\infty}(\mathbb{R}^n)$ as well?

Using the embedding $\dot{H}^1\hookrightarrow L^{p}_{\rho}$ (weighted Lp space) for $p\in [1, 2^*]$ we know that the solution $u$ is integrable. I am trying to combine the fact that $\phi \in L^{1}(\mathbb{R}^n)\cap C^{\alpha}_{\operatorname{loc}}(\mathbb{R}^n)$ to arrive at a contradiction, but this seems not trivial at the moment.

Any references/hints will be much appreciated.

PS. Note that $\dot{H}^1(\mathbb{R}^n)$ is the closure of $C^{\infty}_{c}(\mathbb{R}^n)$ with respect to the semi-norm $\|\nabla u\|^2_{L^2}.$

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  • $\begingroup$ @WillieWong I fixed the embedding, it happens in the weighted Lp space. $\endgroup$
    – Student
    Commented Jul 8, 2021 at 14:22
  • $\begingroup$ One can view $\lambda$ as an eigenvalue and $\phi$ as the corresponding eigenfunction. $\endgroup$
    – Student
    Commented Jul 8, 2021 at 14:22
  • $\begingroup$ By standard elliptic regularity theory, I had De Giorgi-Nash-Moser theorem which works for elliptic equations with bounded and measurable coefficients. $\endgroup$
    – Student
    Commented Jul 8, 2021 at 14:24
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    $\begingroup$ In dimension 3 for example, $u\in \dot{H}^1\implies u \in L^6$, and thus $\rho u \in L^6$ and inverting the Laplacian (you don't need De Giorgi-Nash-Moser here, can get away with boundedness of Riesz transform) gives you $u\in W^{2,6}$ which gives that $u\in C^{1,\alpha}$ (no "loc"). $\endgroup$ Commented Jul 8, 2021 at 14:47

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