1. L. V. Ahlfors, ‘‘Sur le type d’une surface de Riemann,’’ C. R. Acad. Sci. Paris 201, 30–32 (1935).
2. R. Nevanlina, ‘‘Ein Satz über offenen Riemannsche Flächen,’’ Ann. Acad. Sci. Fenn., Part A 54, 1–18 (1940).
3. P. Grigor’yan, ‘‘Analitic and geometric background of recurence and non-explosion of the Brownian motion on Riemannian manifolds,’’ Bull. Am. Math. Soc. 36, 135–249 (1999).
4. S. Y. Cheng and S. T. Yau, ‘‘Differential equations on Riemannian manifolds and their geometric applications,’’ Comm. Pure Appl. Math. 28, 333–354 (1975).
5. A. Grigor’yan, ‘‘On the existance of positive fundamental solution of the Laplace equation on Riemannian manifolds,’’ Mat. Sb. 128, 354–363 (1985).