A Three Critical Points Theorem and Its Applications to the Ordinary Dirichlet Problem

The aim of this paper is twofold. On one hand we establish a three critical points theorem for functionals depending on a real parameter λ ∈ Λ, which is different from the one proved by B. Ricceri in [15] and gives an estimate of where Λ can be located. On the other hand, as an application of the previous result, we prove an existence theorem of three classical solutions for a two-point boundary value problem which is independent from the one by J. Henderson and H. B. Thompson ([10]). Specifically, an example is given where the key assumption of [10] fails. Nevertheless, the existence of three solutions can still be deduced using our theorem.