# Publications of Kristály, Alexandru

A dimension-depending multiplicity result for a perturbed Schrödinger equation

We consider the Schrodinger equation $$ \Delta u + V (x)u = \lambda K(x)f(u) + \mu L(x)g(u) \mbox{ in } R^N; \ u\in H^1(R^N), \eqno{(P)} $$ where $N\ge 2$, $\lambda , \mu \ge 0$ are parameters, $V,K,L : R^N\rightarrow R$ are radially symmetric potentials, $f : R\rightarrow R$ is a continuous function with sublinear growth at infinity, and $g : R\rightarrow R$ is a continuous sub-critical function. We first prove that for $\lambda $ small enough no non-zero solution exists for $(P)$, while for $\lambda $ large and $\mu $ small enough at least two distinct non-zero radially symmetric solutions do exist for $(P)$. By exploiting a Ricceri-type three-critical points theorem, the principle of symmetric criticality and a group-theoretical approach, the existence of at least $N-3$ ($N$ mod 2) distinct pairs of non-zero solutions is guaranteed for $(P)$ whenever $\lambda $ is large and $\mu $ is small enough, $N\neq 3$, and $f, g$ are odd.

New competition phenomena in Dirichlet problems

We study the multiplicity of nonnegative solutions to the problem, (Pλ) where Ω is a smooth bounded domain in RN, f:[0,∞)→R oscillates near the origin or at infinity, and p>0, λ∈R. While oscillatory right-hand sides usually produce infinitely many distinct solutions, an additional term involving up may alter the situation radically. Via a direct variational argument we fully describe this phenomenon, showing that the number of distinct non-trivial solutions to problem (Pλ) is strongly influenced by up and depends on λ whenever one of the following two cases holds: •p⩽1 and f oscillates near the origin; •p⩾1 and f oscillates at infinity (p may be critical or even supercritical). The coefficient a∈L∞(Ω) is allowed to change its sign, while its size is relevant only for the threshold value p=1 when the behaviour of f(s)/s plays a crucial role in both cases. Various - and L∞-norm estimates of solutions are also given.