MAPPING PROPERTIES OF COMBINED FIELD HELMHOLTZ BOUNDARY INTEGRAL OPERATORS

For the Helmholtz equation (with wavenumber k) and analytic boundaries Gamma we analyze the mapping properties of the single layer and double layer as well as combined potential boundary integral operators. A k-explicit regularity theory for the single layer and double layer potentials is developed, in which these operators are decomposed into three parts: the first part is the single or double layer potential for the Laplace equation, the second part is an operator with finite shift properties, and the third part is an operator that maps into a space of piecewise analytic functions. For all parts, the k-dependence is made explicit. We also develop a k-explicit regularity theory for the inverse of the combined potential operator A = +/- 1/2+K-i eta V and its adjoint, where V and K are the single layer and double layer operators for the Helmholtz kernel and eta is an element of R is a coupling parameter with vertical bar eta vertical bar similar to vertical bar k vertical bar. The decomposition of the inverses A(-1) and (A')(-1) takes the form of a sum of two operators A(1), A(2), where A(1) : H-s(Gamma) -> H-s(Gamma) with bounds independent of k and a smoothing operator A(2) that maps into a space of analytic functions on Gamma. The k-dependence of the mapping properties of A(2) is made explicit.