The solution of a multidimensional, many-region boundary value problem is expressed, in each region, by a truncated, rapidly convergent series of functions that are solutions of the partial differential equation in the given region, i.e., “solution-functions”. The eigenvalues and the corresponding eigenfunctions of the problem are then found by a least mean square method. This scheme is applied by means of a computer to the calculation of eigenvalues and eigenfluxes in the diffusion approximation of a nuclear reactor of cylindrical geometry.