Symmetry and regularity of extremals of an integral equation related to the Hardy-Sobolev inequality

Let alpha be a real number satisfying 0 < alpha < n, 0 <= t < alpha, alpha*(t) = 2(n-t)/n-alpha. We consider the integral equation u(x) = integral(Rn)u(alpha*(t)-1)(y)/|y|(t)|x-y|(n-alpha)dy, which is closely related to the Hardy-Sobolev inequality. In this paper, we prove that every positive solution u(x) is radially symmetric and strictly decreasing about the origin by the method of moving plane in integral forms. Moreover, we obtain the regularity of solutions to the following integral equation u(x) = integral(Rn)|u(y)|(p)u(y)/|y|(t)|x-y|n(-alpha)dy (2) that corresponds to a large class of PDEs by regularity lifting method.