Grigory Perelman, who could be rich or famous for proving the Poincar&eaucte; Conjecture, has declined to pick up the Field Medal, the mathematical equivalent of the Nobel Prize, at a ceremony in Madrid for solving a problem first posed in 1904. Silvia Nasar, who wrote A Beautiful Mind about another famous mathematician, interviewed the reclusive 40 year-old Russian for this week's New Yorker magazine.<br>