The Poincaré Conjecture in simple terms “states that the three-sphere is the only type of bounded three-dimensional space possible that contains no holes. This conjecture was first proposed in 1904 by H. Poincaré” – Mathworld
Elusive Proof, Elusive Prover: A New Mathematical Mystery:
But at the moment of his putative triumph, Dr. Perelman is nowhere in sight. He is an odds-on favorite to win a Fields Medal, math’s version of the Nobel Prize, when the International Mathematics Union convenes in Madrid next Tuesday. But there is no indication whether he will show up.
Also left hanging, for now, is $1 million offered by the Clay Mathematics Institute in Cambridge, Mass., for the first published proof of the conjecture, one of seven outstanding questions for which they offered a ransom back at the beginning of the millennium.
The Clay Math Institute site has more on the Poincaré Conjecture. Read about the rest of their Millennium Problems – 6 more problems each with a $1 million prize.

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