Generalized Nehari manifold and semilinear Schrödinger equation with weak monotonicity condition on the nonlinear term

We study the Schrödinger equations $-Δu + V(x)u = f(x,u)$ in $\mathbb{R}^N$ and $-Δu - λu = f(x,u)$ in a bounded domain $Ω\subset\mathbb{R}^N$. We assume that $f$ is superlinear but of subcritical growth and $u\mapsto f(x,u)/|u|$ is nondecreasing. In $\mathbb{R}^N$ we also assume that $V$ and $f$ are periodic in $x_1,\ldots,x_N$. We show that these equations have a ground state and that there exist infinitely many solutions if $f$ is odd in $u$. Our results generalize those in \cite{sw1} where $u\mapsto f(x,u)/|u|$ was assumed to be strictly increasing. This seemingly small change forces us to go beyond methods of smooth analysis.

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