Ground states of a nonlocal variational problem and Thomas-Fermi limit for the Choquard equation
We study nonnegative optimizers of a Gagliardo-Nirenberg type inequality $$\iint_{\mathbb{R}^N \times \mathbb{R}^N} \frac{|u(x)|^p\,|u(y)|^p}{|x - y|^{N-α}} dx\, dy\le C\Big(\int_{{\mathbb R}^N}|u|^2 dx\Big)^{pθ} \Big(\int_{{\mathbb R}^N}|u|^q dx\Big)^{2p(1-θ)/q},$$ that involves the nonlocal Riesz energy with $0<α \frac{N+α}{N}$, $q>\frac{2Np}{N+α}$ and $θ=\frac{(N+α)q-2Np}{Np(q-2)}$. For $p=2$, the equivalent problem has been studied in connection with the Keller-Segel diffusion-aggregation models in the past few decades. The general case $p\neq 2$ considered here appears in the study of Thomas-Fermi limit regime for the Choquard equations with local repulsion. We establish optimal ranges of parameters for the validity of the above interpolation inequality, discuss the existence and qualitative properties of the nonnegative maximizers, and in some special cases estimate the optimal constant. For $p=2$ it is known that the maximizers are Hölder continuous and compactly supported on a ball. We show that for $p<2$ the maximizers are smooth functions supported on $\mathbb{R}^N$, while for $p>2$ the maximizers consist of a characteristic function of a ball and a nonconstant nonincreasing Hölder continuous function supported on the same ball. We use these qualitative properties of the maximizers to establish the validity of the Thomas-Fermi approximations for the Choquard equations with local repulsion. The results are verified numerically with extensive examples.