On the periodic and asymptotically periodic nonlinear Helmholtz equation

In the first part of this paper, the existence of infinitely many $L^p$-standing wave solutions for the nonlinear Helmholtz equation $$-\mathrm{\Delta }u-\lambda u=Q(x)\mathrm{\mid }u{\mathrm{\mid }}^{p-2}u\text{ in }{\mathbb{R}}^N$$ is proven for $N\geq 2$and$\lambda>0$,undertheassumptionthat$Q$ be a nonnegative, periodic and bounded function and the exponent $p$ lies in the Helmholtz subcritical range. In a second part, the existence of a nontrivial solution is shown in the case where the coefficient $Q$ is only asymptotically periodic.