In this talk, we discuss the existence of critical points for a family of abstract and smooth functionals on Banach spaces under the energy constraint. By using the Ljusternick-Schnirelmann theory and the fibering method of Pohozaev we show, under suitable assumptions, multiplicity results.
The abstract framework is then applied to some partial differential equations depending on a parameter for which we obtain multiple solutions as well as some bifurcation results.