A multilevel stochastic-gradient neural solver for boundary integral equations

Abstract: We develop a multilevel stochastic-gradient neural solver for boundary integral equations of the second kind. The unknown density is represented by a multilayer perceptron, trained by minimizing the Nyström-discretized residual on a ladder of refining quadrature grids, each level warm-started from the parameters of the previous one. Each step requires only dense matrix-vector products on mini-batches of collocation rows and network passes, operations that map directly onto GPU hardware. The residual contraction is governed by the empirical neural tangent kernel (NTK), the discrete sample of a single continuum kernel. On a fixed grid, training stalls once the residual concentrates in modes the network contracts slowly, the plateau described by the frequency principle; a spectral analysis explains, and experiments confirm, how refining the quadrature resolves more of the continuum kernel's spectrum and returns these modes to the optimizer's reach. Spectral bias, elsewhere an obstruction to neural network solvers, thus serves as the smoother of a multigrid-type iteration, with quadrature refinement in place of coarse-grid correction. Under a uniform regularity bound on the network, the total work is a constant multiple of the work on the finest grid, and the uniform conditioning of the discrete second-kind operator leaves the NTK as the sole rate-determining spectrum while converting the training residual into an a posteriori error bound. Experiments on interior Dirichlet Laplace/Poisson problems and exterior Neumann Helmholtz problems, using both parametric and signed-distance surface representations, demonstrate the effectiveness and efficiency of the proposed method compared with GMRES at comparable tolerances.