1. Sobolev Spaces;Adams,1975
2. Degree theory for operators of monotone type and nonlinear elliptic equations with inequality constraints;Aizicovici;Mem. Amer. Math. Soc.,2008
3. A Picone's identity for the p-Laplacian and applications;Allegretto;Nonlinear Anal.,1998
4. Infinite-Dimensional Morse Theory and Multiple Solution Problems;Chang,1993
5. Methods in Nonlinear Analysis;Chang,2005