Boundary behaviour of eigenfunctions of the Laplacian in a bi-tree. Peter Sjögren, M.A. Picardello (1992) Journal für die reine und angewandte Mathematik
Boundary-Variation Solution of Eigenvalue Problems for Elliptic Operators. Fernando Reitich, Oscar P. Bruno (2001) The journal of Fourier analysis and applications [[Elektronische Ressource]]