A surface integral equation formulation for low-frequency scattering from a composite object

A surface integral equation formulation is derived for low-frequency scattering from a composite object. This formulation allows those algorithms for finding the loop-tree basis on simple surfaces to be applied to the complicated interfaces of a composite object. By including the residue term in the K operator, the present formulation induces the interface boundary conditions automatically, and the resultant matrix equation can be solved directly without using the O(N/sup 2/) number-of-unknowns-reduction scheme. Numerical results validate that the algorithm is stable even at extremely low frequencies and for closely spaced structures.