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Groundstates of nonlinear Choquard equations: Hardy-Littlewood-Sobolev critical exponent

We consider nonlinear Choquard equation $$ - Δu + V u = \bigl(I_α\ast |u|^{\fracα{N}+1}\bigr) |u|^{\fracα{N}-1} u\quad\text{in (\mathbb{R}^N)},$$ where $N \ge 3$, $V \in L^\infty (\mathbb{R}^N)$ is an external potential and $I_α(x)$ is the Riesz potential of order $α\in (0, N)$. The power $\fracα{N}+1$ in the nonlocal part of the equation is critical with respect to the Hardy-Littlewood-Sobolev inequality. As a consequence, in the associated minimization problem a loss of compactness may occur. We prove that if $\liminf_{|x| \to \infty} \bigl(1 - V (x)\bigr)|x|^2 > \frac{N^2 (N - 2)}{4 (N + 1)}$ then the equation has a nontrivial solution. We also discuss some necessary conditions for the existence of a solution. Our considerations are based on a concentration compactness argument and a nonlocal version of Brezis-Lieb lemma.