On the Expansion of an Arbitrary Function in a Series of Eigenfunctions of Boundary Value Problems II

Let D be a bounded open domain, simply or multiply connected, whose boundary C is composed of a finite number of regular curves in the x,y plane, and let w_n(x,y) be the eigenfunctions (normal and orthogonal) with the corresponding eigenvalues λ_n > 0, n = 1, 2 , . . . of the boundary value problem.