which we will write as operator equation Vf = g where V is a pseudodifferential operator of order -1. This problem is equivalent to the Dirichlet problem for the Laplace equation in the exterior domain with Dirichlet data g on . We are furthermore interested in the hypersingular equation

which we will write as operator equation Df = g where D is a pseudodifferential operator of order 1, and in the double layer potential equation

which corresponds to the interior Neumann problem.

We solve the integral equations with the BEM where we use semi-orthogonal Riesz bases as ansatz functions as well as test functions (Galerkin method). In the numerical tests we use two different examples for such Riesz bases:

• example 1: piecewise constant functions (Haar basis)
• example 2: semi-orthogonal linear spline wavelets

In the following, Figure 1 shows the generating functions for the piecewise constant Riesz bases (Figure 1 a) as well as for the semi-orthogonal linear spline wavelets (Figure 1 b,c). The grid point sets of both above examples of Riesz bases can be seen in Figure 2. Here, the one-dimensional case is shown in the top of the figure. In the middle of this figure the full grid case can be seen, while the sparse grid case is presented at the bottom.

The discretization of each of the above integral equations leads to a linear system of equations where the entries of the stiffness matrices can be computed exactly using recurrence formulae which can be found in [8]. Obviously, the resulting stiffnes matrices are symmetric positive definite as we use the same bases for the ansatz and test functions. Hence, we use the conjugate gradients method as an iterative solver where we use Jacobian preconditioning. We have run comparative test for both above mentioned Riesz bases. All tests are for the capacity problem for a unit square screen with

for the single layer potential equation with right hand side g=1, e.g. we compute approximate solutions of the problem Vf=1. The capacity of a screen is defined as the average of the solution f of the equation Vf=1:

Let us denote by L the Level of the discretization (of a full grid and a a sparse grid, respectively), and let and be the Galerkin solutions of the single layer potential equation for the full grid space and the sparse grid space , respectively. It is well-known that

holds. Therefore, it is enough to concentrate on capacity errors. In the following, some numerical examples are shown. In the tables we use the abbrevations L for Level, K for kondition number of the stiffness matrix, It for the number of iterations to reduce the relative error by 10^-6 in the residual of the preconditioned system, E(C) for the capacity error and q for the quotient in the capacity error showing the numerical convergence of the scheme. First, the results obtained using the Haar basis are presented.

Table 1: Results for tensor product haar bases for full grids (columns 2-5, first columns denoted by K/It/E(C)/q) and sparse grids (columns 6-9, second columns denoted by K/It/E(C)/q)

Level	K	It	E(C)	q	K	It	E(C)	q
1	1.00	1	-0.030452	-	1.00	1	-0.030452	-
2	2.13	3	-0.016812	1.81	1.95	2	-0.016831	1.81
3	3.50	8	-0.009133	1.84	3.00	4	-0.009209	1.83
4	5.39	12	-0.004806	1.90	4.53	8	-0.004915	1.87
5	7.53	14	-0.002483	1.94	6.29	12	-0.002596	1.89
6	-	-	-0.001268	1.96	8.39	14	-0.001369	1.90
7	-	-	-0.000643	1.97	10.79	17	-0.000723	1.89
8	-	-	-0.000325	1.98	13.56	20	-0.000384	1.88

The results of Table 1 can also be seen in the following two figures (Figure 1 and Figure 2) where the capacity error and the condition number divided by L² vs. degrees of freedom can be seen. As the latter expression tends to a constant value, the condition number of the stiffnes matrices is proportional to L² for the Haar bases.

Let us now turn to the results one gets when taking the linear spline prewavelts into account. Table 2 contains the numerical results, again for the full grid case (columns 2-5) and for the sparse grid case (columns 6-9).

Table 2: Results for tensor product linear spline prewavelets for full grids (columns 2-5, first columns denoted by K/It/E(C)/q) and sparse grids (columns 6-9, second columns denoted by K/It/E(C)/q)

Level	K	It	E(C)	q	K	It	E(C)	q
1	1.00	1	-0.030452	-	1.00	1	-0.030452	-
2	35.47	3	-0.013142	2.32	35.42	2	-0.013216	2.30
3	40.18	6	-0.007785	1.69	40.12	4	-0.007839	1.69
4	42.12	9	-0.004113	1.89	42.72	7	-0.004173	1.88
5	-	-	-	-	44.32	9	-0.002169	1.92
6	-	-	-	-	45.29	10	-0.001120	1.94
7	-	-	-	-	45.88	11	-0.000577	1.94

The results of Table 2 can also be seen in the following two figures (Figure 3 and Figure 4) where the capacity error and the condition number vs. degrees of freedom can be seen. Here, we are almost in the asymptotic range, and we already see that the condition number is bounded from above as theory predicts.

The solution of the equation Vf=1 has corner singularities as well as edge singularities. Hence, adaptive refined grids instead of quasiuniform ines are surely more apropriate, compare e.g. [7], [9] and [10]. In the case of the single layer potential equation the leading singular components of the solution f are known. Hence, we use suitable regularized functions that describes the behaviour of the leading singularity to generate an adaptive sparse grid. The whole procedure as well as a regularized function can be found in [2]. Grids obtained with the regularized function in [2] can be found in Figure 5. Here, the maximum level of the grids are 4, 12 and 21, and the corresponding number N of grid points is 57, 217 and 1841.

Figure 5: adaptive sparse grids obtained by the regularized function (L_max = 4, 12, 21, resp. N = 57, 217, 1841)

Taking the above adaptive sparse grids into account, the numerical results using linear spline prewavelets for the Galerkin solution of the single layer potential equation can be found in Table 3. Here, L_max deotes the maximum Level of a grid point in the adaptive sparse grid and N denotes the number of grid points. Again, K denotes the condition number, It the number of CG iterations to reduce the relative error by 10^-6 in the residual of the preconditioned system, and E(C) is the capacity error.

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• [2] M.Griebel, P. Oswald, T. Schiekofer, Sparse grids for boundary integral equations, Numerische Mathematik, submitted, 1998
• [3] M.Griebel, S. Knapek, T. Schiekofer, Remarks on the compression of Galerkin discretizations of Pseudodifferential operators using wavelets of tensor product type, in preparation
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• [5] N. Heuer, M. Maischak, E.P. Stephan , The hp-Version of the Boundary Element Method for Screen Problems, Preprint, Institut für Angewandte Mathematik der Universität Hannover, 1997
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• [8] M. Maischak, hp-Methoden für Randintegralgleichungen bei 3D-Problemen, Theorie und Implementierung, Dissertation, Institut für Angewandte Mathematik der Universität Hannover, 1996
• [9] M. Maischak, P. Mund, E.P. Stephan, Adaptive multilevel BEM for acoustic scattering, Preprint, Institut für Angewandte Mathematik der Universität Hannover, 1997
• [10] P. Mund, E.P. Stephan, Adaptive two-level boundary element methods for the single layer potential in , Preprint, Institut für Angewandte Mathematik der Universität Hannover, 1997
• [11] P. Oswald, Nonlinear N-term approximation of singularity functions in two Haar Bases, Preprint, Bell Labs, Lucent Technologies, 1998

• Compression of Galerkin discretizations of pseudodifferential operators using wavelets of tensor-product type (S. Knapek)