[1] LI Gong-bao, MARTIO O. Local and global integrability of gradients in obstacle problems [J]. Annales Academic Scientiarum Fennicae Mathematica 1994, 19. [2] HEINONEN J, K ILPELAINEN T, MARTIO O. Nonliner potential theory of second order degenerate eliptic partial differential equations [M]. London:Oxford University Press 1993.