AREA INTEGRAL ESTIMATES FOR THE BIHARMONIC OPERATOR IN LIPSCHITZ-DOMAINS

Let D is contained in or equal to R(n) be a Lipschitz domain and let u be a function biharmonic in D, i.e., DELTA-DELTA-u = 0 in D. We prove that the nontangential maximal function and the square function of the gradient of u have equivalent L(p)(d-mu) norms, where d-mu is-an-element-of A(infinity) and d-sigma is surface measure on partial derivative D.