A partial solution to the problem of proper reparametrization for rational surfaces

Given an algebraically closed field K, and a rational parametrization P of an algebraic surface V subset of K-3, we consider the problem of computing a proper rational parametrization from P (reparametrization problem). More precisely, we present an algorithm that computes a rational parametrization Q of V such that the degree of the rational map induced by Q is less than the degree induced by P. The properness of the output parametrization Q is analyzed. In particular, if the degree of the map induced by Q is one, then Q is proper and the reparametrization problem is solved. The algorithm works if at least one of two auxiliary parametrizations defined from P is not proper. (C) 2013 Elsevier B.V. All rights reserved.