Topologists struggled for a century to prove whether every simply connected three-dimensional space was equivalent to a round hypersphere; Grigori Perelman treated the universe like a wrinkled sheet of metal, using heat diffusion equations to smooth out wrinkled geometric singularities. Posted quietly to the arXiv in 2002, Perelman’s proof solved the Poincaré Conjecture—becoming the first and only Millennium Prize Problem ever conquered.

In 1904, French genius Henri Poincaré asked if a loop of string wrapped around any closed three-dimensional space could be pulled tight to a single point, does that space have to be a standard round 3D sphere? For a century, topologists proved it true for every higher dimension (4D, 5D, 6D), but three dimensions remained stubbornly unsolved.
Russian mathematical recluse Grigori Perelman solved it by treating space like an expanding ball of heat. Using "Ricci flow," he flowed heat across wrinkled 3D geometry to smooth out rough spots, performing delicate mathematical surgery with scissors to snip off pinching necks before they blew up into singularities.
Perelman proved that every 3-dimensional universe must round out into a sphere. By solving the 100-year-old Poincaré Conjecture, by proving Thurston’s Geometrization of all 3-manifolds, and by famously declining the $1 million Clay Millennium Prize, Perelman conquered modern topology.
The entropy formula for the Ricci flow and its geometric applications
We present a monotonic expression for the Ricci flow, valid in all dimensions and without curvature assumptions. It is interpreted as an entropy for a certain canonical ensemble. Several geometric applications are given. In particular, (1) Ricci flow, considered on the space of riemannian metrics modulo diffeomorphism and scaling, has no nontrivial periodic orbits (that is, other than fixed points); (2) In a region, where singularity is forming in finite time, the injectivity radius is controlled by the curvature; (3) Ricci flow can not quickly turn an almost euclidean region into a very curved one, no matter what happens far away. We also verify several assertions related to Richard Hamilton's program for the proof of Thurston geometrization conjecture for closed three-manifolds, and give a sketch of an eclectic proof of this conjecture, making use of earlier results on collapsing with local lower curvature bound.
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