  This article is dedicated to research of approximation properties of
B-splines and Lagrangian finite elements in Hilbert spaces of functions defined
on surfaces in three-dimensional space. Hereinafter the conditions are
determined for convergence of Galerkin and collocation methods for solving
Fredholm integral equations of the first kind for simple layer potential that
is equivalent to Dirichlet problem for Laplace equation in R3. Estimation is
determined for the error of approximate solution of this problem obtained using
potential theory methods.
