Andrei Gabrielov Nicolai Vorobjov Betti Numbers of Semialgebraic Sets Defined by Quantifier-Free Formulae. 395-401 2005 33 Discrete & Computational Geometry 3 http://www.springerlink.com/index/10.1007/s00454-004-1105-7 db/journals/dcg/dcg33.html#GabrielovV05