A million-dollar prize for solving one of toughest problems in mathematics has been awarded to a Russian mathematician, but the real puzzle is whether he’ll accept it.

The reclusive Grigori Perelman has been recognised for his proof of the Poincaré conjecture, one of seven Millennium prize problems selected by the Clay Mathematics Institute (CMI) in 2000 as the most important unsolved problems in mathematics.

The conjecture, proposed by Henri Poincaré in 1904, deals with the properties of spheres in three dimensions. If we tie a loop around a two-dimensional sphere, such as the skin of an orange, it’s always possible to shrink it down to a point. The same isn’t true of a doughnut, as a loop passing through the central hole cannot be shrunk. Poincaré asserted this was also true in three dimensions for a sphere.

Perelman published a proof in 2002, but since became disillusioned with mathematics and withdrew from the mathematical community. In 2006 he refused to accept a Fields medal for his work, an award often described as the Nobel prize of mathematics.

The president of CMI, James Carlson, is waiting to see if Perelman will do the same for the Millennium prize. “It may be a while before he makes his decision,” he says. The Poincaré conjecture is the only one of the seven Millennium problems that has been solved to date, and the fate of the prize money is uncertain if Perelman rejects.