The boundary integral equation method in conjunction with the degenerate kernel, the direct searching technique (singular value decomposition) and the only two-trials technique ( 22  matrix eigenvalue problem) are analytically and numerically used to find the degenerate scales, respectively. In the continuous system of boundary integral equation, the degenerate kernel for the 2D Kelvin solution in the polar coordinates is reviewed and the degenerate kernel in the elliptical coordinates is derived. Using the degenerate kernel, analytical solution of the degenerate scales for elasticity problem of circular and elliptical cases obtained and compared with the numerical result. Besides, the triangular case and square case were also numerically demonstrated.

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In the development of finite element method (FEM), the patch test is required. We may wonder whether we need any special test for the boundary element method (BEM). A sufficient and necessary boundary integral equation method (BIEM) to ensure a unique solution is our concern. In this paper, we revisit this issue for the interior two-dimensional (2-D) elasticity problem and investigate the equivalence of solution space between the integral equation and the partial differential equation. Based on the degenerate kernel and the eigenfunction expansion, the range deficiency of the integral operator of the single-layer potential for the solution space in the degenerate-scale problem for the 2-D elasticity in the BIEM is analytically studied. Following the Fichera's idea, we enrich the conventional BIEM by adding constants and corresponding constraints. In addition, we introduce the concept of modal participation factor (MPF) to examine whether the adding term of the rotation is required for interior simply-connected problems. Finally, a simple example of the degenerate-scale problem containing an elliptical boundary subjected to various boundary conditions of the rigid body translation and rotation for 2-D elasticity problems was demonstrated by using the necessary and sufficient BIEM.

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