On Palais' principle for nonsmooth functionals

If G is a compact Lie group acting linearly on a Banach space X and f is a G-invariant function on X, we provide some new versions of the so-called Palais’ principle of symmetric criticality for f : X → R, in the framework of non-smooth critical point theory. We apply the results to a class of quasi-linear PDEs associated with invariant functionals which are merely lower semi-continuous and thus could not be treated by previous non-smooth versions of the principle in the literature.