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The spurious eigenvalues of an annular domain have been veri› ed for the singular and hypersingular boundary-element methods (BEMs) and circumvented by using the Burton{Miller approach. Do they also occur in other formulations: continuous formulations such as the singular and hypersingular boundary integral equations (BIEs), the null-› eld BIEs and the › ctitious BIEs, or such discrete formulations as the null-› eld BEMs and the › ctitious BEMs? For the ten formulations of the multiply connected problem the study of otherwise the same issues is continued in the present paper. By using the degenerate kernels and the Fourier series, it is demonstrated analytically for the six continuous formulations of BIEs that spurious eigensolutions depend on the geometry of the inner boundary but not on that of the outer boundary. This conclusion can be extended to the six discrete formulations of BEMs. To › lter out the spurious eigenvalues, the CHIEF (combined Helmholtz integral equation for- mulation) method is used here instead of the Burton{Miller approach. The optimum number and appropriate positions of the CHIEF points are also addressed. It is then shown that, in the null-› eld and › ctitious BEMs, the spurious and true eigenval- ues can be detected and distinguished by using the singular-value-decomposition- updating techniques in conjunction with the Fredholm alternative theorem. Illustra- tive examples show the validity of the proposed methodologies.

Spurious and true eigensolutions of Helmholtz BIEs and BEMs for a multiply connected problem