Which mathematician solved the Poincare conjecture and refused the Fields Medal?

Henri Poincare asked in 1904 whether every closed three-dimensional space with no holes in it must essentially be a sphere. The question resisted for a century, was named one of the seven Millennium Prize Problems in 2000 with a million dollars attached, and remains the only one solved.

Grigori Perelman settled it between 2002 and 2003 in three preprints posted to the arXiv. He did not submit them to a journal, did not write an introduction explaining their importance, and in places left the reader to fill in the argument. The method built on Richard Hamilton's Ricci flow, which smooths a space out the way heat spreads through metal; the obstacle had always been that the flow can tear, and Perelman's contribution was to understand the tears well enough to cut them out and keep going. It took other mathematicians three years to verify.

He was awarded the Fields Medal in 2006 and turned it down - the only person ever to do so. In 2010 he refused the million dollars as well, on the grounds that Hamilton had contributed no less than he had and that a prize going to one of them was simply unjust. He withdrew from mathematics altogether and has lived quietly in Saint Petersburg since. It is unusual to be able to point at a person and say the incentives did not apply.