nonlinearity in $\mathbb {R}^ N $\begin {equation*}\sqrt {\strut-\Delta+ m^ 2} u-mu+ V (x)
u=\left (\int_ {\mathbb {R}^ N}\frac {| u (y)|^ p}{| xy|^{N-\alpha}}\, dy\right)| u|^{p-2} u-\Gamma
(x)| u|^{q-2} u,\end {equation*} where $ m> 0$ and the potential $ V $ is decomposed as the
sum of a $\mathbb {Z}^ N $-periodic term and of a bounded term that decays at infinity. The
result is proved by variational methods applied to an auxiliary problem in the half-space …