1. Ambrosetti, A. 1992.Critical points and nonlinear variational problems, Vol. 120, 5–139. France: Suppl. Bull. Soc. Math.
2. Brezis, H. and Nirenberg, L. 1993.H1versus C1local minimizers, Vol. 317, 465–472. Paris: C.R. Acad. Sci.
3. Clement, P. and Sweers, G. 1987.Existence and multiplicity results for a semilinear elliptic eigenvalue problem, Vol. 14, 97–121. Pisa: Annali Scuola Norm. Sup.