Riemann and Klein Surfaces with Nodes Viewed as Quotients

Résumé

If G is a group of automorphisms that acts properly discontinuously on a Riemann or Klein surface X, then there exists aunique structure of Riemann orKlein surface on X/G such that the projection π : X → X/G is a morphism.The analogousresult is not true when we deal with surfaces with nodes. In thispaper we give a new definition of a group that acts properly discontinuously on a surface with nodes in order to obtain a similar theorem.