Non-negative solutions of a sublinear elliptic problem

Abstract

In this paper the existence of solutions, $(\lambda,u)$, of the problem $$-\Delta u=\lambda u -a(x)|u|^{p-1}u \quad \hbox{in }\Omega, \qquad u=0 \quad \hbox{on}\;\;\partial\Omega,$$ is explored for $0 < p < 1$. When $p>1$, it is known that there is an unbounded component of such solutions bifurcating from $(\sigma_1, 0)$, where $\sigma_1$ is the smallest eigenvalue of $-\Delta$ in $\Omega$ under Dirichlet boundary conditions on $\partial\Omega$. These solutions have $u \in P$, the interior of the positive cone. The continuation argument used when $p>1$ to keep $u \in P$ fails if $0 < p < 1$. Nevertheless when $0 < p < 1$, we are still able to show that there is a component of solutions bifurcating from $(\sigma_1, \infty)$, unbounded outside of a neighborhood of $(\sigma_1, \infty)$, and having $u \gneq 0$. This non-negativity for $u$ cannot be improved as is shown via a detailed analysis of the simplest autonomous one-dimensional version of the problem: its set of non-negative solutions possesses a countable set of components, each of them consisting of positive solutions with a fixed (arbitrary) number of bumps. Finally, the structure of these components is fully described.