Maximal existence domains of positive solutions for two-parametric systems of elliptic equations

The paper is devoted to the study of two-parametric families of Dirichlet problems for systems of equations with $p, q$-Laplacians and indefinite nonlinearities. Continuous and monotone curves $\Gamma_f$ and $\Gamma_e$ on the parametric plane $\lambda \times \mu$, which are the lower and upper bounds for a maximal domain of existence of weak positive solutions are introduced. The curve $\Gamma_f$ is obtained by developing our previous work \cite{BobkovIlyasov} and it determines a maximal domain of the applicability of the Nehari manifold and fibering methods. The curve $\Gamma_e$ is derived explicitly via minimax variational principle of the extended functional method.