[1]  Arcoya D, Boccardo L. Critical points for multiple integral of the calculus of variations. Arch Rational Mech Anal, 1996, 134: 249--274

[2] Bartsch T, Li Shujie. Critical point theory for asymptotically quadratic functionals and applications to problems with resonance. Nonlinear Anal, 1997, 28(3): 4l9--441

[3] Bensoussan A, Boccardo L,  Murat F. On a nonlinear partial differential equation having natural growth and unbounded solutions. Ann Inst H Poincarè Anal Non Linèaire, 1988, 5: 347--364

[4] Canino A. Multiplicity of solutions for quasi-linear elliptic equations. Top Meth Nonlin Anal, 1995, 6: 357--370

[5]  Canino A, Degiovanni M. Non-smooth critical point theory and quasi-linear elliptic equations//Granras A, Frigon M, Sabidussi G, eds.
Topological Methods in Differential Equations and Inclusions. NATO ASJ Series-Kluwer A P, 1995: 1--50

[6]  Cao D M, Yan S S. Infinitely many solutions for an elliptic problem involving critical nonlinearity. Acta Math Sci, 2010, 30B(6): 2017--2032

[7]  Corvellec J N. Nontrivial solutions of quasi-linear equations via non-smooth Morse theory. J Differ Equa, 1997, 136: 268--293

[8]  Corvellec J N. Morse Theory for continuous functionals. J Math Anal Appl, 1995, 196: 1050--1072

[9]  Degiovanni M, Marzocchi M. A critical point theory for non-smooth functionals.  Ann Mat Pura Appl, 1994, 167(4): 73--100

[10]  Evans L C. Partial Differential Equations. Providence RI:  American Mathematical Society, 1998

[11]  Guo Y X, Liu J Q. Solutions of p-sublinear p-Laplacian equation via Morse theory. J London Math Soc, 2005, 72(2): 632--644

[12]  Moroz V. Solutions of super-linear at zero elliptic equations via Morse theory. Top  Meth Nonlin Anal, 1997, 10: 387--397

[13] Palais R. Morse theory on Hilbert manifolds. Topology, 1963, 2: 299--340

[14]  MacLane S J R. Elements of Algebraic Topology. Reading, Ma: Addison-Wesley, Perseus, 1993

[15] Shen Y T, Guo X K.  Applications of the three critical points theorem in quasilinear elliptic equations. Acta Math Sci, 1985, 5(3): 279--288

[16]  Smale S. Morse theory and a nonlinear generalization of the Dirichlet problem. Ann Math, 1964, 80: 382--396

[17] Spanier E H. Algebraic Topology. New York: McGraw-Hill Book Co, 1966

[18]  Squalsina M. Existence of weak solutions to general Euler's equations via non-smooth critical point theory. Ann Fac Sci Toulouse Math, 2000, 9(6): 113--131