“…The study of regularity estimates in various function spaces for linear or non-linear elliptic equations (or systems) in non-smooth domains is one of the most interesting and important topics in partial differential equations (see, for instance, [1,7,13,17,26,29,32,33,43,44] for the linear case and [2,3,10,18,19,21,51,52] for the non-linear case). Furthermore, it is well known that the global regularity estimates for solutions to elliptic boundary problems depend not only on the structure of equations and the properties of the right-hand side datum and the coefficients appearing in equations, but also on the smooth property or the geometric property of the boundary of domains (see, for instance, [1,7,16,21,29,33,44,48,51]).…”