Solutions of super-linear elliptic equations and their Morse indices

Abstract: We investigate here the degenerate bi-harmonic equation: $$\Delta_{m}^2 u=f(x,u)\; \;\;\mbox{in} \O,\quad u = \Delta u = 0\quad \mbox{on }\; \p\Omega,$$ with $m\ge 2,$ and also the degenerate tri-harmonic equation: $$ -\Delta_{m}^3 u=f(x,u)\;\;\; \mbox{in} \O,\quad u = \frac{\p u}{\p \nu} = \frac{\p^{2} u}{\p\nu^{2}} = 0\quad \mbox{on }\; \p\Omega,$$ where $\Omega\subset \mathbb{R}^{N}$ is a bounded domain with smooth boundary $N>4$ or $N>6$ resp, and $f \in \mathrm{C}^{1}(\Omega\times \mathbb{R})$ satisfying suitable m-superlinear and subcritical growth conditions. Our main purpose is to establish $L^{p}$ and $L^{\infty}$ explicit bounds for weak solutions via the Morse index. Our results extend previous explicit estimate obtained in \cite{c, HHF, hyf, lec}.