[1]  Meyers N G, Elcrat A. Some results on regularity for solutions of nonlinear elliptic systems and quasiregular functions. Duke Math J, 1975, 42: 121--136

[2]  Li G B, Martio O. Local and global integrability of gradients in obstacle problems. Ann Acad Sci Fenn Ser A I Math, 1994, 19: 25--34

[3]  Gao H Y, Tian H Y. Local regularity result for solutions of obstacle problems. Acta Math Sci, 2004, 24B(1): 71--74

[4]  Giaquinta M, Giusti E. On the regularity of the minima of variational integrals. Acta Math, 1982, 148(1):  31--46

[5]  Giachetti D, Porzio M M. Local regularity results for minima of functionals of the calculus of variation. Nonlinear
Analysis, TMA, 2000, 39: 463--482

[6]  Giaquinta M. Multiple Integrals in the Calculus of Variations and Nonlinear Elliptic Systems. Princeton: Princeton Univ Press, 1983

[7]  Gilbarg D, Trudinger N S. Elliptic Partial Differential Equations of Second Order. Grundlehren der
Mathemaischen Wissenschaften, Vol   224. 2nd ed. Berlin: Springer-Verlag, 1983

[8] Gao H Y, He Q, Niu H L, Chu Y M. Local regularity for very weak solutions of obstacle problems to A-harmonic equations. Acta Math Sci, 2009, 29A(5): 1291--1297