This is an early access version, the complete PDF, HTML, and XML versions will be available soon. Open AccessArticle by Chiara Brambilla Chiara Brambilla Scilit Preprints.org Google Scholar , Luca Grosset Luca Grosset Scilit Preprints.org Google Scholar * and Elena Sartori Elena Sartori Scilit Preprints.org Google Scholar Department of Mathematics “Tullio Levi-Civita”, University of Padova, 35121 Padova, Italy * Author to whom correspondence should be addressed.