Grigori Perelman proves the Poincare conjecture

In November 2002 and March and July 2003, the Russian mathematician Grigori Perelman posted three papers on the online archive arXiv which solved the Poincare conjecture, a problem posed in 1904 by Henri Poincare about the nature of three-dimensional spheres, and one of the seven Millennium Prize Problems of the Clay Mathematics Institute with a prize of one million dollars. Building on Richard Hamilton's work on Ricci flow, he also proved the more general geometrisation conjecture of William Thurston. Perelman declined the Fields Medal in 2006 and the Clay prize in 2010, and withdrew from the mathematical community. It is the only Millennium problem solved so far.

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