In the paper, results on the existence of critical points in annular subsets of a cone are obtained with the additional goal of obtaining multiplicity results. Compared to other approaches in the literature based on the use of Krasnoselskii's compression-extension theorem or topological index methods, our approach uses the Nehari manifold technique in a surprising combination with the cone version of Birkhoff-Kellogg's invariant-direction theorem. This yields a simpler alternative to traditional methods involving deformation arguments or Ekeland variational principle. The new method is illustrated on a boundary value problem for p-Laplacian equations, and we believe that it will be useful for proving the existence, localization, and multiplicity of solutions for other classes of problems with variational structure.

Manuscript accepted for publication. Clarifications in Theorem 3.2, modification of Lemma 3.1