Poincaré conjecture
Field  Geometric topology 

Conjectured by  Henri Poincaré 
Conjectured in  1904 
First proof by  Grigori Perelman 
First proof in  2006 
Implied by  
Equivalent to  
Generalizations  Generalized Poincaré conjecture 
Millennium Prize Problems 

In mathematics, the Poincaré conjecture ( UK: /ˈpwæ̃kæreɪ/,^{ [2]} US: /ˌpwæ̃kɑːˈreɪ/,^{ [3]}^{ [4]} French: [pwɛ̃kaʁe]) is a theorem about the characterization of the 3sphere, which is the hypersphere that bounds the unit ball in fourdimensional space.
The conjecture states:
Every simply connected, closed 3 manifold is homeomorphic to the 3sphere.
An equivalent form of the conjecture involves a coarser form of equivalence than homeomorphism called homotopy equivalence: if a 3manifold is homotopy equivalent to the 3sphere, then it is necessarily homeomorphic to it.
Originally conjectured by Henri Poincaré, the theorem concerns a space that locally looks like ordinary threedimensional space but is connected, finite in size, and lacks any boundary (a closed 3manifold). The Poincaré conjecture claims that if such a space has the additional property that each loop in the space can be continuously tightened to a point, then it is necessarily a threedimensional sphere. The analogous conjectures for all higher dimensions were proved before a proof of the original conjecture was found.
After nearly a century of effort by mathematicians, Grigori Perelman presented a proof of the conjecture in three papers made available in 2002 and 2003 on arXiv. The proof built upon the program of Richard S. Hamilton to use the Ricci flow to attempt to solve the problem. Hamilton later introduced a modification of the standard Ricci flow, called Ricci flow with surgery to systematically excise singular regions as they develop, in a controlled way, but was unable to prove this method "converged" in three dimensions.^{ [5]} Perelman completed this portion of the proof. Several teams of mathematicians verified that Perelman's proof was correct.
The Poincaré conjecture, before being proved, was one of the most important open questions in topology. In 2000, it was named one of the seven Millennium Prize Problems, for which the Clay Mathematics Institute offered a $1 million prize for the first correct solution. Perelman's work survived review and was confirmed in 2006, leading to his being offered a Fields Medal, which he declined. Perelman was awarded the Millennium Prize on March 18, 2010.^{ [6]} On July 1, 2010, he turned down the prize, saying that he believed his contribution in proving the Poincaré conjecture was no greater than Hamilton's.^{ [7]}^{ [8]} As of 2021^{ [update]}, the Poincaré conjecture is the only solved Millennium problem.
On December 22, 2006, the journal Science honored Perelman's proof of the Poincaré conjecture as the scientific " Breakthrough of the Year", the first time this honor was bestowed in the area of mathematics.^{ [9]}
History
Poincaré's question
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At the beginning of the 20th century, Henri Poincaré was working on the foundations of topology—what would later be called combinatorial topology and then algebraic topology. He was particularly interested in what topological properties characterized a sphere.
Poincaré claimed in 1900 that homology, a tool he had devised based on prior work by Enrico Betti, was sufficient to tell if a 3manifold was a 3sphere. However, in a 1904 paper he described a counterexample to this claim, a space now called the Poincaré homology sphere. The Poincaré sphere was the first example of a homology sphere, a manifold that had the same homology as a sphere, of which many others have since been constructed. To establish that the Poincaré sphere was different from the 3sphere, Poincaré introduced a new topological invariant, the fundamental group, and showed that the Poincaré sphere had a fundamental group of order 120, while the 3sphere had a trivial fundamental group. In this way he was able to conclude that these two spaces were, indeed, different.
In the same paper, Poincaré wondered whether a 3manifold with the homology of a 3sphere and also trivial fundamental group had to be a 3sphere. Poincaré's new condition—i.e., "trivial fundamental group"—can be restated as "every loop can be shrunk to a point."
The original phrasing was as follows:
Consider a compact 3dimensional manifold V without boundary. Is it possible that the fundamental group of V could be trivial, even though V is not homeomorphic to the 3dimensional sphere?
Poincaré never declared whether he believed this additional condition would characterize the 3sphere, but nonetheless, the statement that it does is known as the Poincaré conjecture. Here is the standard form of the conjecture:
Every simply connected, closed 3 manifold is homeomorphic to the 3sphere.
Note that "closed" here means, as customary in this area, the condition of being compact in terms of set topology, and also without boundary (3dimensional Euclidean space is an example of a simply connected 3manifold not homeomorphic to the 3sphere; but it is not compact and therefore not a counterexample).
Attempted solutions
This problem seemed to lie dormant until J. H. C. Whitehead revived interest in the conjecture, when in the 1930s he first claimed a proof and then retracted it. In the process, he discovered some examples of simplyconnected (indeed contractible, i.e. homotopically equivalent to a point) noncompact 3manifolds not homeomorphic to , the prototype of which is now called the Whitehead manifold.
In the 1950s and 1960s, other mathematicians attempted proofs of the conjecture only to discover that they contained flaws. Influential mathematicians such as Georges de Rham, R. H. Bing, Wolfgang Haken, Edwin E. Moise, and Christos Papakyriakopoulos attempted to prove the conjecture. In 1958 Bing proved a weak version of the Poincaré conjecture: if every simple closed curve of a compact 3manifold is contained in a 3ball, then the manifold is homeomorphic to the 3sphere.^{ [10]} Bing also described some of the pitfalls in trying to prove the Poincaré conjecture.^{ [11]}
Włodzimierz Jakobsche showed in 1978 that, if the Bing–Borsuk conjecture is true in dimension 3, then the Poincaré conjecture must also be true.^{ [12]}
Over time, the conjecture gained the reputation of being particularly tricky to tackle. John Milnor commented that sometimes the errors in false proofs can be "rather subtle and difficult to detect."^{ [13]} Work on the conjecture improved understanding of 3manifolds. Experts in the field were often reluctant to announce proofs, and tended to view any such announcement with skepticism. The 1980s and 1990s witnessed some wellpublicized fallacious proofs (which were not actually published in peerreviewed form).^{ [14]}^{ [15]}
An exposition of attempts to prove this conjecture can be found in the nontechnical book Poincaré's Prize by George Szpiro.^{ [16]}
Dimensions
The classification of closed surfaces gives an affirmative answer to the analogous question in two dimensions. For dimensions greater than three, one can pose the Generalized Poincaré conjecture: is a homotopy nsphere homeomorphic to the nsphere? A stronger assumption is necessary; in dimensions four and higher there are simplyconnected, closed manifolds which are not homotopy equivalent to an nsphere.
Historically, while the conjecture in dimension three seemed plausible, the generalized conjecture was thought to be false. In 1961 Stephen Smale shocked mathematicians by proving the Generalized Poincaré conjecture for dimensions greater than four and extended his techniques to prove the fundamental hcobordism theorem. In 1982 Michael Freedman proved the Poincaré conjecture in four dimensions. Freedman's work left open the possibility that there is a smooth fourmanifold homeomorphic to the foursphere which is not diffeomorphic to the foursphere. This socalled smooth Poincaré conjecture, in dimension four, remains open and is thought to be very difficult. Milnor's exotic spheres show that the smooth Poincaré conjecture is false in dimension seven, for example.
These earlier successes in higher dimensions left the case of three dimensions in limbo. The Poincaré conjecture was essentially true in both dimension four and all higher dimensions for substantially different reasons. In dimension three, the conjecture had an uncertain reputation until the geometrization conjecture put it into a framework governing all 3manifolds. John Morgan wrote:^{ [17]}
It is my view that before Thurston's work on hyperbolic 3manifolds and . . . the Geometrization conjecture there was no consensus among the experts as to whether the Poincaré conjecture was true or false. After Thurston's work, notwithstanding the fact that it had no direct bearing on the Poincaré conjecture, a consensus developed that the Poincaré conjecture (and the Geometrization conjecture) were true.
Hamilton's program and Perelman's solution
Hamilton's program was started in his 1982 paper in which he introduced the Ricci flow on a manifold and showed how to use it to prove some special cases of the Poincaré conjecture.^{ [18]} In the following years he extended this work, but was unable to prove the conjecture. The actual solution was not found until Grigori Perelman published his papers.
In late 2002 and 2003 Perelman posted three papers on the arXiv.^{ [19]}^{ [20]}^{ [21]} In these papers he sketched a proof of the Poincaré conjecture and a more general conjecture, Thurston's geometrization conjecture, completing the Ricci flow program outlined earlier by Richard S. Hamilton.
From May to July 2006, several groups presented papers that filled in the details of Perelman's proof of the Poincaré conjecture, as follows:
 Bruce Kleiner and John W. Lott posted a paper on the arXiv in May 2006 which filled in the details of Perelman's proof of the geometrization conjecture, following partial versions which had been publicly available since 2003.^{ [22]} Their manuscript was published in the journal "Geometry and Topology" in 2008. A small number of corrections were made in 2011 and 2013; for instance, the first version of their published paper made use of an incorrect version of Hamilton's compactness theorem for Ricci flow.
 HuaiDong Cao and XiPing Zhu published a paper in the June 2006 issue of the Asian Journal of Mathematics with an exposition of the complete proof of the Poincaré and geometrization conjectures.^{ [23]} The opening paragraph of their paper stated
In this paper, we shall present the HamiltonPerelman theory of Ricci flow. Based on it, we shall give the first written account of a complete proof of the Poincaré conjecture and the geometrization conjecture of Thurston. While the complete work is an accumulated efforts of many geometric analysts, the major contributors are unquestionably Hamilton and Perelman.
 Some observers interpreted Cao and Zhu as taking credit for Perelman's work. They later posted a revised version, with new wording, on the arXiv.^{ [24]} In addition, a page of their exposition was essentially identical to a page in one of Kleiner and Lott's early publicly available drafts; this was also amended in the revised version, together with an apology by the journal's editorial board.
 John Morgan and Gang Tian posted a paper on the arXiv in July 2006 which gave a detailed proof of just the Poincaré Conjecture (which is somewhat easier than the full geometrization conjecture)^{ [25]} and expanded this to a book.^{ [26]} In 2015, Abbas Bahri pointed out that pages 441445 of Morgan and Tian's exposition were incorrect.^{ [27]} The error was later fixed by Morgan and Tian.^{ [28]}
All three groups found that the gaps in Perelman's papers were minor and could be filled in using his own techniques.
On August 22, 2006, the ICM awarded Perelman the Fields Medal for his work on the conjecture, but Perelman refused the medal.^{ [29]}^{ [30]}^{ [31]} John Morgan spoke at the ICM on the Poincaré conjecture on August 24, 2006, declaring that "in 2003, Perelman solved the Poincaré Conjecture."^{ [32]}
In December 2006, the journal Science honored the proof of Poincaré conjecture as the Breakthrough of the Year and featured it on its cover.^{ [9]}
Ricci flow with surgery
Hamilton's program for proving the Poincaré conjecture involves first putting a Riemannian metric on the unknown simply connected closed 3manifold. The basic idea is to try to "improve" this metric; for example, if the metric can be improved enough so that it has constant positive curvature, then according to classical results in Riemannian geometry, it must be the 3sphere. Hamilton prescribed the " Ricci flow equations" for improving the metric;
where g is the metric and R its Ricci curvature, and one hopes that as the time t increases the manifold becomes easier to understand. Ricci flow expands the negative curvature part of the manifold and contracts the positive curvature part.
In some cases Hamilton was able to show that this works; for example, his original breakthrough was to show that if the Riemannian manifold has positive Ricci curvature everywhere, then the above procedure can only be followed for a bounded interval of parameter values, with , and more significantly, that there are numbers such that as , the Riemannian metrics smoothly converge to one of constant positive curvature. According to classical Riemannian geometry, the only simplyconnected compact manifold which can support a Riemannian metric of constant positive curvature is the sphere. So, in effect, Hamilton showed a special case of the Poincaré conjecture: if a compact simplyconnected 3manifold supports a Riemannian metric of positive Ricci curvature, then it must be diffeomorphic to the 3sphere.
If, instead, one only has an arbitrary Riemannian metric, the Ricci flow equations must lead to more complicated singularities. Perelman's major achievement was to show that, if one takes a certain perspective, if they appear in finite time, these singularities can only look like shrinking spheres or cylinders. With a quantitative understanding of this phenomenon, he cuts the manifold along the singularities, splitting the manifold into several pieces, and then continues with the Ricci flow on each of these pieces. This procedure is known as Ricci flow with surgery.
Perelman provided a separate argument based on curve shortening flow to show that, on a simplyconnected compact 3manifold, any solution of the Ricci flow with surgery becomes extinct in finite time. An alternative argument, based on the minmax theory of minimal surfaces and geometric measure theory, was provided by Tobias Colding and William Minicozzi. Hence, in the simplyconnected context, the above finitetime phenomena of Ricci flow with surgery is all that is relevant. In fact, this is even true if the fundamental group is a free product of finite groups and cyclic groups.
This condition on the fundamental group turns out to be necessary and sufficient for finite time extinction. It is equivalent to saying that the prime decomposition of the manifold has no acyclic components, and turns out to be equivalent to the condition that all geometric pieces of the manifold have geometries based on the two Thurston geometries S^{2}×R and S^{3}. In the context that one makes no assumption about the fundamental group whatsoever, Perelman made a further technical study of the limit of the manifold for infinitely large times, and in so doing, proved Thurston's geometrization conjecture: at large times the manifold has a thickthin decomposition, whose thick piece has a hyperbolic structure, and whose thin piece is a graph manifold. Due to Perelman's and Colding and Minicozzi's results, however, these further results are unnecessary in order to prove the Poincaré conjecture.
Solution
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On November 13, 2002, Russian mathematician Grigori Perelman posted the first of a series of three eprints on arXiv outlining a solution of the Poincaré conjecture. Perelman's proof uses a modified version of a Ricci flow program developed by Richard S. Hamilton. In August 2006, Perelman was awarded, but declined, the Fields Medal (worth $15,000 CAD) for his proof. On March 18, 2010, the Clay Mathematics Institute awarded Perelman the $1 million Millennium Prize in recognition of his proof.^{ [33]}^{ [34]} Perelman rejected that prize as well.^{ [7]}^{ [35]}
Perelman proved the conjecture by deforming the manifold using the Ricci flow (which behaves similarly to the heat equation that describes the diffusion of heat through an object). The Ricci flow usually deforms the manifold towards a rounder shape, except for some cases where it stretches the manifold apart from itself towards what are known as singularities. Perelman and Hamilton then chop the manifold at the singularities (a process called "surgery") causing the separate pieces to form into balllike shapes. Major steps in the proof involve showing how manifolds behave when they are deformed by the Ricci flow, examining what sort of singularities develop, determining whether this surgery process can be completed and establishing that the surgery need not be repeated infinitely many times.
The first step is to deform the manifold using the Ricci flow. The Ricci flow was defined by Richard S. Hamilton as a way to deform manifolds. The formula for the Ricci flow is an imitation of the heat equation which describes the way heat flows in a solid. Like the heat flow, Ricci flow tends towards uniform behavior. Unlike the heat flow, the Ricci flow could run into singularities and stop functioning. A singularity in a manifold is a place where it is not differentiable: like a corner or a cusp or a pinching. The Ricci flow was only defined for smooth differentiable manifolds. Hamilton used the Ricci flow to prove that some compact manifolds were diffeomorphic to spheres and he hoped to apply it to prove the Poincaré Conjecture. He needed to understand the singularities.^{[ citation needed]}
Hamilton created a list of possible singularities that could form but he was concerned that some singularities might lead to difficulties. He wanted to cut the manifold at the singularities and paste in caps, and then run the Ricci flow again, so he needed to understand the singularities and show that certain kinds of singularities do not occur. Perelman discovered the singularities were all very simple: essentially threedimensional cylinders made out of spheres stretched out along a line. An ordinary cylinder is made by taking circles stretched along a line. Perelman proved this using something called the "Reduced Volume" which is closely related to an eigenvalue of a certain elliptic equation.
Sometimes an otherwise complicated operation reduces to multiplication by a scalar (a number). Such numbers are called eigenvalues of that operation. Eigenvalues are closely related to vibration frequencies and are used in analyzing a famous problem: can you hear the shape of a drum? Essentially an eigenvalue is like a note being played by the manifold. Perelman proved this note goes up as the manifold is deformed by the Ricci flow. This helped him eliminate some of the more troublesome singularities that had concerned Hamilton, particularly the cigar soliton solution, which looked like a strand sticking out of a manifold with nothing on the other side. In essence Perelman showed that all the strands that form can be cut and capped and none stick out on one side only.
Completing the proof, Perelman takes any compact, simply connected, threedimensional manifold without boundary and starts to run the Ricci flow. This deforms the manifold into round pieces with strands running between them. He cuts the strands and continues deforming the manifold until eventually he is left with a collection of round threedimensional spheres. Then he rebuilds the original manifold by connecting the spheres together with threedimensional cylinders, morphs them into a round shape and sees that, despite all the initial confusion, the manifold was in fact homeomorphic to a sphere.
One immediate question posed was how one could be sure that infinitely many cuts weren't necessary. This was raised due to the cutting potentially progressing forever. Perelman proved this can't happen by using minimal surfaces on the manifold. A minimal surface is essentially a soap film. Hamilton had shown that the area of a minimal surface decreases as the manifold undergoes Ricci flow. Perelman verified what happened to the area of the minimal surface when the manifold was sliced. He proved that eventually the area is so small that any cut after the area is that small can only be chopping off threedimensional spheres and not more complicated pieces. This is described as a battle with a Hydra by Sormani in Szpiro's book cited below. This last part of the proof appeared in Perelman's third and final paper on the subject.
References
 ^ Matveev, Sergei (2007). "1.3.4 Zeeman's Collapsing Conjecture". Algorithmic Topology and Classification of 3Manifolds. Algorithms and Computation in Mathematics. 9. Springer. pp. 46–58. ISBN 9783540458999.
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^
"Prize for Resolution of the Poincaré Conjecture Awarded to Dr. Grigoriy Perelman" (Press release).
Clay Mathematics Institute. March 18, 2010. Archived from
the original (PDF) on March 22, 2010. Retrieved November 13, 2015.
The Clay Mathematics Institute (CMI) announces today that Dr. Grigoriy Perelman of St. Petersburg, Russia, is the recipient of the Millennium Prize for resolution of the Poincaré conjecture.
 ^ ^{a} ^{b} "Последнее "нет" доктора Перельмана" [The last "no" Dr. Perelman]. Interfax (in Russian). July 1, 2010. Retrieved 5 April 2016. Google Translated archived link at [1] (archived 20140420)
 ^ Ritter, Malcolm (1 July 2010). "Russian mathematician rejects million prize". The Boston Globe.
 ^ ^{a} ^{b} Mackenzie, Dana (20061222). "The Poincaré Conjecture—Proved". Science. 314 (5807): 1848–1849. doi: 10.1126/science.314.5807.1848. PMID 17185565.
 ^ Bing, R. H. (1958). "Necessary and sufficient conditions that a 3manifold be S^{3}". Annals of Mathematics. Second Series. 68 (1): 17–37. doi: 10.2307/1970041. JSTOR 1970041.
 ^ Bing, R. H. (1964). "Some aspects of the topology of 3manifolds related to the Poincaré conjecture". Lectures on Modern Mathematics. II. New York: Wiley. pp. 93–128.
 ^ M., Halverson, Denise; Dušan, Repovš (23 December 2008). "The Bing–Borsuk and the Busemann conjectures". Mathematical Communications. 13 (2). arXiv: 0811.0886.
 ^ Milnor, John (2004). "The Poincaré Conjecture 99 Years Later: A Progress Report" (PDF). Retrieved 20070505.
 ^ Taubes, Gary (July 1987). "What happens when hubris meets nemesis". Discover. 8: 66–77.
 ^ Matthews, Robert (9 April 2002). "$1 million mathematical mystery "solved"". NewScientist.com. Retrieved 20070505.
 ^ Szpiro, George (July 29, 2008). Poincaré's Prize: The HundredYear Quest to Solve One of Math's Greatest Puzzles. Plume. ISBN 9780452289642.
 ^ Morgan, John W., Recent progress on the Poincaré conjecture and the classification of 3manifolds. Bull. Amer. Math. Soc. (N.S.) 42 (2005), no. 1, 57–78
 ^ Hamilton, Richard (1982). "Threemanifolds with positive Ricci curvature". Journal of Differential Geometry. 17 (2): 255–306. doi: 10.4310/jdg/1214436922. MR 0664497. Zbl 0504.53034. Reprinted in: Cao, H. D.; Chow, B.; Chu, S. C.; Yau, S.T., eds. (2003). Collected Papers on Ricci Flow. Series in Geometry and Topology. 37. Somerville, MA: International Press. pp. 119–162. ISBN 1571461108.
 ^ Perelman, Grigori (2002). "The entropy formula for the Ricci flow and its geometric applications". arXiv: math.DG/0211159.
 ^ Perelman, Grigori (2003). "Ricci flow with surgery on threemanifolds". arXiv: math.DG/0303109.
 ^ Perelman, Grigori (2003). "Finite extinction time for the solutions to the Ricci flow on certain threemanifolds". arXiv: math.DG/0307245.
 ^ Kleiner, Bruce; John W. Lott (2008). "Notes on Perelman's Papers". Geometry and Topology. 12 (5): 2587–2855. arXiv: math.DG/0605667. doi: 10.2140/gt.2008.12.2587. S2CID 119133773.
 ^ Cao, HuaiDong; XiPing Zhu (June 2006). "A Complete Proof of the Poincaré and Geometrization Conjectures – application of the HamiltonPerelman theory of the Ricci flow" (PDF). Asian Journal of Mathematics. 10 (2). Archived from the original (PDF) on 20120514.
 ^ Cao, HuaiDong & Zhu, XiPing (December 3, 2006). "Hamilton–Perelman's Proof of the Poincaré Conjecture and the Geometrization Conjecture". arXiv: math.DG/0612069.
 ^ Morgan, John; Gang Tian (2006). "Ricci Flow and the Poincaré Conjecture". arXiv: math.DG/0607607.
 ^ Morgan, John; Gang Tian (2007). Ricci Flow and the Poincaré Conjecture. Clay Mathematics Institute. ISBN 9780821843284.
 ^ Bahri, Abbas (2015). "Five gaps in mathematics". Adv. Nonlinear Stud. 15 (2): 289–319. doi: 10.1515/ans20150202. S2CID 125566270.
 ^ Morgan, John; Tian, Gang (2015). "Correction to Section 19.2 of Ricci Flow and the Poincare Conjecture". arXiv: 1512.00699 [ math.DG].
 ^ Nasar, Sylvia; David Gruber (August 28, 2006). "Manifold destiny". The New Yorker. pp. 44–57. Online version at the New Yorker website.
 ^ Chang, Kenneth (August 22, 2006). "Highest Honor in Mathematics Is Refused". The New York Times.
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 ^ A Report on the Poincaré Conjecture. Special lecture by John Morgan.
 ^ "Prize for Resolution of the Poincaré Conjecture Awarded to Dr. Grigoriy Perelman". Clay Mathematics Institute. March 18, 2010. Archived from the original on 20100322.
 ^ "Poincaré Conjecture". Clay Mathematics Institute. Retrieved 20181004.
 ^ Malcolm Ritter (20100701). "Russian mathematician rejects $1 million prize". Phys.Org. Retrieved 20110515.
Further reading
 Kleiner, Bruce; Lott, John (2008). "Notes on Perelman's papers". Geometry & Topology. 12 (5): 2587–2855. arXiv: math/0605667. doi: 10.2140/gt.2008.12.2587. MR 2460872. S2CID 119133773.
 HuaiDong Cao; XiPing Zhu (December 3, 2006). "HamiltonPerelman's Proof of the Poincaré Conjecture and the Geometrization Conjecture". arXiv: math.DG/0612069.
 Morgan, John W.; Tian, Gang (2007). Ricci Flow and the Poincaré Conjecture. Clay Mathematics Monographs. 3. Providence, RI: American Mathematical Society. arXiv: math/0607607. ISBN 9780821843284. MR 2334563.: Detailed proof, expanding Perelman's papers.
 O'Shea, Donal (December 26, 2007). The Poincaré Conjecture: In Search of the Shape of the Universe. Walker & Company. ISBN 9780802716545.
 Perelman, Grisha (November 11, 2002). "The entropy formula for the Ricci flow and its geometric applications". arXiv: math.DG/0211159.
 Perelman, Grisha (March 10, 2003). "Ricci flow with surgery on threemanifolds". arXiv: math.DG/0303109.
 Perelman, Grisha (July 17, 2003). "Finite extinction time for the solutions to the Ricci flow on certain threemanifolds". arXiv: math.DG/0307245.
 Szpiro, George (July 29, 2008). Poincaré's Prize: The HundredYear Quest to Solve One of Math's Greatest Puzzles. Plume. ISBN 9780452289642.
 Stillwell, John (2012). "Poincaré and the early history of 3manifolds". Bulletin of the American Mathematical Society. 49 (4): 555–576. doi: 10.1090/S02730979201201385X. MR 2958930.
 Yau, ShingTung; Nadis, Steve (2019). The Shape of a Life: One Mathematician's Search for the Universe's Hidden Geometry. New Haven, CT: Yale University Press. ISBN 9780300235906. MR 3930611.
External links
 Poincaré conjecture at ProofWiki
 "The Poincaré Conjecture" – BBC Radio 4 programme In Our Time, 2 November 2006. Contributors June BarrowGreen, Lecturer in the History of Mathematics at the Open University, Ian Stewart, Professor of Mathematics at the University of Warwick, Marcus du Sautoy, Professor of Mathematics at the University of Oxford, and presenter Melvyn Bragg.