  An emended and improved version of the present paper has been archived in
math-ph/0505057, and a preliminary account of its content has been published in
Phys.Rev.Lett. 92, 60601, (2004). Moreover, in order to prove the relevance of
topology for phase transition phenomena in a broad domain of physically
interesting cases, we have proved another theorem which is reported in
math-ph/0505058 and which is crucially based on the result of the paper
archived in math-ph/0505057.
