[1]  Ambrosetti A, Badiale M. Homoclinics: Poincare-Melnikov type results via a variational approach. Ann Inst H Poincare Analyse Non Lineaire, 1998, 15: 233--252

[2]  Ambrosetti A, Badiale M. Variational perturbative methods and bifurcation of bound states from the essential spectrum. Proc Royal Soc Edinburgh, 1998, 128A: 1131--1161

[3]  Ambrosetti A, Garcia Azorero J, Peral I. Remarks on a class of semilinear elliptic equations on R^N, via perturbation methods. Advanced Nonlinear Studies, 2001, 1(1):  1--13

[4]  Abdelkander Boucherif, Mohamed Bouguima. Perturbation in a free boundary problem. Acta Math Sci, 1997, 17(4): 452--465;  J Mathematical Analysis and Applications, 1994, 185: 430--437

[5]  Lions P L. The concentration-compactness principle in the calculus of variations.  The limit case Part 1; and Part 2, Rev Mathematical Iberoamericana, 1985, 1-1: 541--597; 1985, 1-2: 45--121

[6]  Flucher M,  Wei J. Asymptotic shape and location of small cores in elliptic free-boundary problems. Math Z, 1998, 228:  683--703

[7]  Manuel Del Pino, Patricio Felmer. Semi-classical states of nonlinear Schrodinger equations: a variational reduction method.  Mathematische Annalen, 2002, 324: 1--32

[8]  Massimo Grossi. On the number of single-peak solutions of the nonlinear Schrodinger equation. Ann I Poincare, 2002, 19(3): 261--280

[9]  Daomin Cao, Shuangjie Peng, Shusen Yan. Multiplicity of solutions for the plasma problem in two dimensions. to appear

[10]  Yi Li, Shuangjie Peng. Multiple solutions to an elliptic problem related to vortex pairs. to appear