1. Chang, K.C.: Variational methods for non-differentiable functionals and their applications to partial differential equations. J. Math. Anal. Appl. 80, 102–129 (1981)
2. Chang, K.C.: Critical Point Theory and its Applications. Shanghai Scientific and Technological Literature Publishing House, Shanghai (1986)
3. Chang, K.C.: On the multiple solutions of the elliptic differential equations with discontinuous nonlinear terms. Sci. Sin. 21, 139–158 (1978)
4. Chang, K.C.: The obstacle problem and partial differential equations with discontinuous nonlinearities. Commun. Pure Appl. Math. 33, 117–146 (1980)
5. Chang, K.C., Jiang, L.S.: The free boundary problems of the stationary water cone. Sci. Nat. 1, 1–25 (1978)