1. Borel, A., and A. Haefliger: La Classe D'Homologie Fondamentale d'un Espace Analytique. Bull. soc. math. France89, 461?513 (1961).
2. Gunning, R. C., and H. Rossi: Analytic functions of serveral complex variables. Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1965.
3. Lefschetz, S.: Topology. American Mathematical Society, New York 1930.
4. Remmert, R., and K. Stein: �ber die wesentlichen Singularit�ten analytischer Mengen. Math. Ann.126, 263?306 (1953).
5. Samuel, P., and O. Zariski: Commutative algebra II. New York: D. van Nostrand 1958.