A reverse Holder inequality for extremal Sobolev functions

Let $n \geq 2$, let $Ω\subset \mathbf{R}^n$ be a bounded domain with smooth boundary, and let $1 \leq p \leq 2$. We prove a reverse-Holder inequality for functions $u$ realizing the best constant in the Sobolev inequality, that is $\mathcal{C}_p(Ω) = \inf \left \{ \frac{\int_Ω|\nabla v|^2}{\left ( \int_Ω|v|^p \right )^{2/p}} \right \} = \frac{\int_Ω|\nabla u|^2}{\left ( \int_Ω|u|^p \right )^{2/p}}.$ Our inequality has the form $\| u \|_{L^p} \geq K \| u \|_{L^q}$ for any $q > p$, where $K$ depends only on $n$, $p$, $q$, and $\mathcal{C}_p(Ω)$. This result generalizes work of Chiti, regarding the first Dirichlet eigenfunction of the Laplacian, and of van den Berg, regarding the torsion function.

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