# List of conjectures

This is a list of notable mathematical conjectures.

## Open problems

The following conjectures remain open. The (incomplete) column "cites" lists the number of results for a Google Scholar search for the term, in double quotes as of September 2022.
| Conjecture | Field | Comments | Eponym(s) | Cites |
| 1/3–2/3 conjecture | order theory | | n/a | 70 |
| abc conjecture | number theory | ⇔Granville–Langevin conjecture, Vojta's conjecture in dimension 1 ⇒Erdős–Woods conjecture, Fermat–Catalan conjecture Formulated by David Masser and Joseph Oesterlé. Proof claimed in 2012 by Shinichi Mochizuki | n/a | 2440 |
| Agoh–Giuga conjecture | number theory | | Takashi Agoh and Giuseppe Giuga | 8 |
| Agrawal's conjecture | number theory | | Manindra Agrawal | 10 |
| Andrews–Curtis conjecture | combinatorial group theory | | James J. Andrews and Morton L. Curtis | 358 |
| Andrica's conjecture | number theory | | Dorin Andrica | 45 |
| Artin conjecture (L-functions) | number theory | | Emil Artin | 650 |
| Artin's conjecture on primitive roots | number theory | ⇐generalized Riemann hypothesis ⇐Selberg conjecture B | Emil Artin | 325 |
| Bateman–Horn conjecture | number theory | | Paul T. Bateman and Roger Horn | 245 |
| Baum–Connes conjecture | operator K-theory | ⇒Gromov-Lawson-Rosenberg conjecture ⇒Kaplansky-Kadison conjecture ⇒Novikov conjecture | Paul Baum and Alain Connes | 2670 |
| Beal's conjecture | number theory | | Andrew Beal | 142 |
| Beilinson conjecture | number theory | | Alexander Beilinson | 461 |
| Berry–Tabor conjecture | geodesic flow | | Michael Berry and Michael Tabor | 239 |
| Big-line-big-clique conjecture | discrete geometry | | | |
| Birch and Swinnerton-Dyer conjecture | number theory | | Bryan John Birch and Peter Swinnerton-Dyer | 2830 |
| Birch–Tate conjecture | number theory | | Bryan John Birch and John Tate | 149 |
| Birkhoff conjecture | integrable systems | | George David Birkhoff | 345 |
| Bloch–Beilinson conjectures | number theory | | Spencer Bloch and Alexander Beilinson | 152 |
| Bloch–Kato conjecture | algebraic K-theory | | Spencer Bloch and Kazuya Kato | 1620 |
| Bochner–Riesz conjecture | harmonic analysis | ⇒restriction conjecture⇒Kakeya maximal function conjecture⇒Kakeya dimension conjecture | Salomon Bochner and Marcel Riesz | 236 |
| Bombieri–Lang conjecture | diophantine geometry | | Enrico Bombieri and Serge Lang | 181 |
| Borel conjecture | geometric topology | | Armand Borel | 981 |
| Bost conjecture | geometric topology | | Jean-Benoît Bost | 65 |
| Brennan conjecture | complex analysis | | James E. Brennan | 110 |
| Brocard's conjecture | number theory | | Henri Brocard | 16 |
| Brumer–Stark conjecture | number theory | | Armand Brumer and Harold Stark | 208 |
| Bunyakovsky conjecture | number theory | | Viktor Bunyakovsky | 43 |
| Carathéodory conjecture | differential geometry | | Constantin Carathéodory | 173 |
| Carmichael totient conjecture | number theory | | Robert Daniel Carmichael | |
| Casas-Alvero conjecture | polynomials | | Eduardo Casas-Alvero | 56 |
| Catalan–Dickson conjecture on aliquot sequences | number theory | | Eugène Charles Catalan and Leonard Eugene Dickson | 46 |
| Catalan's Mersenne conjecture | number theory | | Eugène Charles Catalan | |
| Cherlin–Zilber conjecture | group theory | | Gregory Cherlin and Boris Zilber | 86 |
| Chowla conjecture | Möbius function | ⇒Sarnak conjecture | Sarvadaman Chowla | |
| Collatz conjecture | number theory | | Lothar Collatz | 1440 |
| Cramér's conjecture | number theory | | Harald Cramér | 32 |
| Conway's thrackle conjecture | graph theory | | John Horton Conway | 150 |
| Deligne conjecture | monodromy | | Pierre Deligne | 788 |
| Dittert conjecture | combinatorics | | Eric Dittert | 11 |
| Eilenberg−Ganea conjecture | algebraic topology | | Samuel Eilenberg and Tudor Ganea | 96 |
| Elliott–Halberstam conjecture | number theory | | Peter D. T. A. Elliott and Heini Halberstam | 300 |
| Erdős–Faber–Lovász conjecture | graph theory | | Paul Erdős, Vance Faber, and László Lovász | 172 |
| Erdős–Gyárfás conjecture | graph theory | | Paul Erdős and András Gyárfás | 37 |
| Erdős–Straus conjecture | number theory | | Paul Erdős and Ernst G. Straus | 103 |
| Farrell–Jones conjecture | geometric topology | | F. Thomas Farrell and Lowell E. Jones | 545 |
| Filling area conjecture | differential geometry | | n/a | 60 |
| Firoozbakht's conjecture | number theory | | Farideh Firoozbakht | 33 |
| Fortune's conjecture | number theory | | Reo Fortune | 16 |
| Four exponentials conjecture | number theory | | n/a | 110 |
| Frankl conjecture | combinatorics | | Péter Frankl | 83 |
| Gauss circle problem | number theory | | Carl Friedrich Gauss | 553 |
| Gilbert–Pollack conjecture on the Steiner ratio of the Euclidean plane | metric geometry | | Edgar Gilbert and Henry O. Pollak | |
| Gilbreath conjecture | number theory | | Norman Laurence Gilbreath | 34 |
| Goldbach's conjecture | number theory | ⇒The ternary Goldbach conjecture, which was the original formulation. | Christian Goldbach | 5880 |
| Gold partition conjecture | order theory | | n/a | 25 |
| Goldberg–Seymour conjecture | graph theory | | Mark K. Goldberg and Paul Seymour | 57 |
| Goormaghtigh conjecture | number theory | | René Goormaghtigh | 14 |
| Green's conjecture | algebraic curves | | Mark Lee Green | 150 |
| Grimm's conjecture | number theory | | Carl Albert Grimm | 46 |
| Grothendieck–Katz p-curvature conjecture | differential equations | | Alexander Grothendieck and Nick Katz | 98 |
| Hadamard conjecture | combinatorics | | Jacques Hadamard | 858 |
| Herzog–Schönheim conjecture | group theory | | Marcel Herzog and Jochanan Schönheim | 44 |
| Hilbert–Smith conjecture | geometric topology | | David Hilbert and Paul Althaus Smith | 219 |
| Hodge conjecture | algebraic geometry | | W. V. D. Hodge | 2490 |
| Homological conjectures in commutative algebra | commutative algebra | | n/a | |
| Hopf conjectures | geometry | | Heinz Hopf | 476 |
| Ibragimov–Iosifescu conjecture for φ-mixing sequences | probability theory | | Ildar Ibragimov, ro:Marius Iosifescu | |
| Invariant subspace problem | functional analysis | | n/a | 2120 |
| Jacobian conjecture | polynomials | | Carl Gustav Jacob Jacobi (by way of the Jacobian determinant) | 2860 |
| Jacobson's conjecture | ring theory | | Nathan Jacobson | 127 |
| Kaplansky conjectures | ring theory | | Irving Kaplansky | 466 |
| Keating–Snaith conjecture | number theory | | Jonathan Keating and Nina Snaith | 48 |
| Köthe conjecture | ring theory | | Gottfried Köthe | 167 |
| Kung–Traub conjecture | iterative methods | | H. T. Kung and Joseph F. Traub | 332 |
| Legendre's conjecture | number theory | | Adrien-Marie Legendre | 110 |
| Lemoine's conjecture | number theory | | Émile Lemoine | 13 |
| Lenstra–Pomerance–Wagstaff conjecture | number theory | | Hendrik Lenstra, Carl Pomerance, and Samuel S. Wagstaff Jr. | 32 |
| Leopoldt's conjecture | number theory | | Heinrich-Wolfgang Leopoldt | 773 |
| List coloring conjecture | graph theory | | n/a | 300 |
| Littlewood conjecture | diophantine approximation | ⇐Margulis conjecture | John Edensor Littlewood | 1230 |
| Lovász conjecture | graph theory | | László Lovász | 560 |
| MNOP conjecture | algebraic geometry | | n/a | 63 |
| Manin conjecture | diophantine geometry | | Yuri Manin | 338 |
| Marshall Hall's conjecture | number theory | | Marshall Hall, Jr. | 44 |
| Mazur's conjectures | diophantine geometry | | Barry Mazur | 97 |
| Montgomery's pair correlation conjecture | number theory | | Hugh Lowell Montgomery | 77 |
| n conjecture | number theory | | n/a | 126 |
| New Mersenne conjecture | number theory | | Marin Mersenne | 47 |
| Novikov conjecture | algebraic topology | | Sergei Novikov | 3090 |
| Oppermann's conjecture | number theory | | Ludvig Oppermann | 12 |
| Petersen coloring conjecture | graph theory | | Julius Petersen | 52 |
| Pierce–Birkhoff conjecture | real algebraic geometry | | Richard S. Pierce and Garrett Birkhoff | 96 |
| Pillai's conjecture | number theory | | Subbayya Sivasankaranarayana Pillai | 33 |
| De Polignac's conjecture | number theory | | Alphonse de Polignac | 46 |
| Quantum PCP conjecture | quantum information theory | | | |
| quantum unique ergodicity conjecture | dynamical systems | 2004, Elon Lindenstrauss, for arithmetic hyperbolic surfaces, 2008, Kannan Soundararajan & Roman Holowinsky, for holomorphic forms of increasing weight for Hecke eigenforms on noncompact arithmetic surfaces | n/a | 281 |
| Reconstruction conjecture | graph theory | | n/a | 1040 |
| Riemann hypothesis | number theory | ⇐Generalized Riemann hypothesis⇐Grand Riemann hypothesis ⇔De Bruijn–Newman constant=0 ⇒density hypothesis, Lindelöf hypothesis See Hilbert–Pólya conjecture. For other Riemann hypotheses, see the Weil conjectures (now theorems). | Bernhard Riemann | 24900 |
| Ringel–Kotzig conjecture | graph theory | | Gerhard Ringel and Anton Kotzig | 187 |
| Rudin's conjecture | additive combinatorics | | Walter Rudin | 16 |
| Sarnak conjecture | topological entropy | | Peter Sarnak | 295 |
| Sato–Tate conjecture | number theory | | Mikio Sato and John Tate | 1080 |
| Schanuel's conjecture | number theory | | Stephen Schanuel | 329 |
| Schinzel's hypothesis H | number theory | | Andrzej Schinzel | 49 |
| Scholz conjecture | addition chains | | Arnold Scholz | 41 |
| Second Hardy–Littlewood conjecture | number theory | | G. H. Hardy and John Edensor Littlewood | 30 |
| Selfridge's conjecture | number theory | | John Selfridge | 6 |
| Sendov's conjecture | complex polynomials | | Blagovest Sendov | 77 |
| Serre's multiplicity conjectures | commutative algebra | | Jean-Pierre Serre | 221 |
| Singmaster's conjecture | binomial coefficients | | David Singmaster | 8 |
| Standard conjectures on algebraic cycles | algebraic geometry | | n/a | 234 |
| Tate conjecture | algebraic geometry | | John Tate | |
| Toeplitz' conjecture | Jordan curves | | Otto Toeplitz | |
| Tuza's conjecture | graph theory | | Zsolt Tuza | |
| Twin prime conjecture | number theory | | n/a | 1700 |
| Ulam's packing conjecture | packing | | Stanislaw Ulam | |
| Unicity conjecture for Markov numbers | number theory | | Andrey Markov (by way of Markov numbers) | |
| Uniformity conjecture | diophantine geometry | | n/a | |
| Unique games conjecture | number theory | | n/a | |
| Vandiver's conjecture | number theory | | Ernst Kummer and Harry Vandiver | |
| Virasoro conjecture | algebraic geometry | | Miguel Ángel Virasoro | |
| Vizing's conjecture | graph theory | | Vadim G. Vizing | |
| Vojta's conjecture | number theory | ⇒abc conjecture | Paul Vojta | |
| Waring's conjecture | number theory | | Edward Waring | |
| Weight monodromy conjecture | algebraic geometry | | n/a | |
| Weinstein conjecture | periodic orbits | | Alan Weinstein | |
| Whitehead conjecture | algebraic topology | | J. H. C. Whitehead | |
| Zauner's conjecture | operator theory | | Gerhard Zauner | |

## Conjectures now proved (theorems)

The conjecture terminology may persist: theorems often enough may still be referred to as conjectures, using the anachronistic names.
| Priority date | Proved by | Former name | Field | Comments |
| 1962 | Walter Feit and John G. Thompson | Burnside conjecture that, apart from cyclic groups, finite simple groups have even order | finite simple groups | Feit–Thompson theorem⇔trivially the "odd order theorem" that finite groups of odd order are solvable groups |
| 1968 | Gerhard Ringel and John William Theodore Youngs | Heawood conjecture | graph theory | Ringel-Youngs theorem |
| 1971 | Daniel Quillen | Adams conjecture | algebraic topology | On the J-homomorphism, proposed 1963 by Frank Adams |
| 1973 | Pierre Deligne | Weil conjectures | algebraic geometry | ⇒Ramanujan–Petersson conjecture Proposed by André Weil. Deligne's theorems completed around 15 years of work on the general case. |
| 1975 | Henryk Hecht and Wilfried Schmid | Blattner's conjecture | representation theory for semisimple groups | |
| 1975 | William Haboush | Mumford conjecture | geometric invariant theory | Haboush's theorem |
| 1976 | Kenneth Appel and Wolfgang Haken | Four color theorem | graph colouring | Traditionally called a "theorem", long before the proof. |
| 1976 | Daniel Quillen; and independently by Andrei Suslin | Serre's conjecture on projective modules | polynomial rings | Quillen–Suslin theorem |
| 1977 | Alberto Calderón | Denjoy's conjecture | rectifiable curves | A result claimed in 1909 by Arnaud Denjoy, proved by Calderón as a by-product of work on Cauchy singular operators |
| 1978 | Roger Heath-Brown and Samuel James Patterson | Kummer's conjecture on cubic Gauss sums | equidistribution | |
| 1983 | Gerd Faltings | Mordell conjecture | number theory | ⇐Faltings's theorem, the Shafarevich conjecture on finiteness of isomorphism classes of abelian varieties. The reduction step was by Alexey Parshin. |
| 1983 onwards | Neil Robertson and Paul D. Seymour | Wagner's conjecture | graph theory | Now generally known as the graph minor theorem. |
| 1983 | Michel Raynaud | Manin–Mumford conjecture | diophantine geometry | The Tate–Voloch conjecture is a quantitative (diophantine approximation) derived conjecture for p-adic varieties. |
| c.1984 | Collective work | Smith conjecture | knot theory | Based on work of William Thurston on hyperbolic structures on 3-manifolds, with results by William Meeks and Shing-Tung Yau on minimal surfaces in 3-manifolds, also with Hyman Bass, Cameron Gordon, Peter Shalen, and Rick Litherland, written up by Bass and John Morgan. |
| 1984 | Louis de Branges de Bourcia | Bieberbach conjecture, 1916 | complex analysis | ⇐Robertson conjecture⇐Milin conjecture⇐de Branges's theorem |
| 1984 | Gunnar Carlsson | Segal's conjecture | homotopy theory | |
| 1984 | Haynes Miller | Sullivan conjecture | classifying spaces | Miller proved the version on mapping BG to a finite complex. |
| 1987 | Grigory Margulis | Oppenheim conjecture | diophantine approximation | Margulis proved the conjecture with ergodic theory methods. |
| 1989 | Vladimir I. Chernousov | Weil's conjecture on Tamagawa numbers | algebraic groups | The problem, based on Siegel's theory for quadratic forms, submitted to a long series of case analysis steps. |
| 1990 | Ken Ribet | epsilon conjecture | modular forms | |
| 1992 | Richard Borcherds | Conway–Norton conjecture | sporadic groups | Usually called monstrous moonshine |
| 1994 | David Harbater and Michel Raynaud | Abhyankar's conjecture | algebraic geometry | |
| 1994 | Andrew Wiles | Fermat's Last Theorem | number theory | ⇔The modularity theorem for semistable elliptic curves. Proof completed with Richard Taylor. |
| 1994 | Fred Galvin | Dinitz conjecture | combinatorics | |
| 1995 | Doron Zeilberger | Alternating sign matrix conjecture, | enumerative combinatorics | |
| 1996 | Vladimir Voevodsky | Milnor conjecture | algebraic K-theory | Voevodsky's theorem, ⇐norm residue isomorphism theorem⇔Beilinson–Lichtenbaum conjecture, Quillen–Lichtenbaum conjecture. The ambiguous term "Bloch-Kato conjecture" may refer to what is now the norm residue isomorphism theorem. |
| 1998 | Thomas Callister Hales | Kepler conjecture | sphere packing | |
| 1998 | Thomas Callister Hales and Sean McLaughlin | dodecahedral conjecture | Voronoi decompositions | |
| 2000 | Krzysztof Kurdyka, Tadeusz Mostowski, and Adam Parusiński | Gradient conjecture | gradient vector fields | Attributed to René Thom, c.1970. |
| 2001 | Christophe Breuil, Brian Conrad, Fred Diamond and Richard Taylor | Taniyama–Shimura conjecture | elliptic curves | Now the modularity theorem for elliptic curves. Once known as the "Weil conjecture". |
| 2001 | Mark Haiman | n! conjecture | representation theory | |
| 2001 | Daniel Frohardt and Kay Magaard | Guralnick–Thompson conjecture | monodromy groups | |
| 2002 | Preda Mihăilescu | Catalan's conjecture, 1844 | exponential diophantine equations | ⇐Pillai's conjecture⇐abc conjecture Mihăilescu's theorem |
| 2002 | Maria Chudnovsky, Neil Robertson, Paul D. Seymour, and Robin Thomas | strong perfect graph conjecture | perfect graphs | Chudnovsky–Robertson–Seymour–Thomas theorem |
| 2002 | Grigori Perelman | Poincaré conjecture, 1904 | 3-manifolds | |
| 2003 | Grigori Perelman | geometrization conjecture of Thurston | 3-manifolds | ⇒spherical space form conjecture |
| 2003 | Ben Green; and independently by Alexander Sapozhenko | Cameron–Erdős conjecture | sum-free sets | |
| 2003 | Nils Dencker | Nirenberg–Treves conjecture | pseudo-differential operators | |
| 2004 (see comment) | Nobuo Iiyori and Hiroshi Yamaki | Frobenius conjecture | group theory | A consequence of the classification of finite simple groups, completed in 2004 by the usual standards of pure mathematics. |
| 2004 | Adam Marcus and Gábor Tardos | Stanley–Wilf conjecture | permutation classes | Marcus–Tardos theorem |
| 2004 | Ualbai U. Umirbaev and Ivan P. Shestakov | Nagata's conjecture on automorphisms | polynomial rings | |
| 2004 | Ian Agol; and independently by Danny Calegari–David Gabai | tameness conjecture | geometric topology | ⇒Ahlfors measure conjecture |
| 2008 | Avraham Trahtman | Road coloring conjecture | graph theory | |
| 2008 | Chandrashekhar Khare and Jean-Pierre Wintenberger | Serre's modularity conjecture | modular forms | |
| 2009 | Jeremy Kahn and Vladimir Markovic | surface subgroup conjecture | 3-manifolds | ⇒Ehrenpreis conjecture on quasiconformality |
| 2009 | Jeremie Chalopin and Daniel Gonçalves | Scheinerman's conjecture | intersection graphs | |
| 2010 | Terence Tao and Van H. Vu | circular law | random matrix theory | |
| 2011 | Joel Friedman; and independently by Igor Mineyev | Hanna Neumann conjecture | group theory | |
| 2012 | Simon Brendle | Hsiang–Lawson's conjecture | differential geometry | |
| 2012 | Fernando Codá Marques and André Neves | Willmore conjecture | differential geometry | |
| 2013 | Yitang Zhang | bounded gap conjecture | number theory | The sequence of gaps between consecutive prime numbers has a finite lim inf. See Polymath Project#Polymath8 for quantitative results. |
| 2013 | Adam Marcus, Daniel Spielman and Nikhil Srivastava | Kadison–Singer problem | functional analysis | The original problem posed by Kadison and Singer was not a conjecture: its authors believed it false. As reformulated, it became the "paving conjecture" for Euclidean spaces, and then a question on random polynomials, in which latter form it was solved affirmatively. |
| 2015 | Jean Bourgain, Ciprian Demeter, and Larry Guth | Main conjecture in Vinogradov's mean-value theorem | analytic number theory | Bourgain–Demeter–Guth theorem, ⇐ decoupling theorem |
| 2018 | Karim Adiprasito | g-conjecture | combinatorics | |
| 2019 | Dimitris Koukoulopoulos and James Maynard | Duffin–Schaeffer conjecture | number theory | Rational approximation of irrational numbers |
* Deligne's conjecture on 1-motives
* Goldbach's weak conjecture (proved in 2013)
* Sensitivity conjecture (proved in 2019)

## Disproved (no longer conjectures)

The conjectures in following list were not necessarily generally accepted as true before being disproved.
* Atiyah conjecture (not a conjecture to start with)
* Borsuk's conjecture
* Chinese hypothesis (not a conjecture to start with)
* Doomsday conjecture
* Euler's sum of powers conjecture
* Ganea conjecture
* Generalized Smith conjecture
* Hauptvermutung
* Hedetniemi's conjecture, counterexample announced 2019
* Hirsch conjecture (disproved in 2010)
* Intersection graph conjecture
* Kelvin's conjecture
* Kouchnirenko's conjecture
* Mertens conjecture
* Pólya conjecture, 1919 (1958)
* Ragsdale conjecture
* Schoenflies conjecture (disproved 1910)
* Tait's conjecture
* Von Neumann conjecture
* Weyl–Berry conjecture
* Williamson conjecture
In mathematics, ideas are supposedly not accepted as fact until they have been rigorously proved. However, there have been some ideas that were fairly accepted in the past but which were subsequently shown to be false. The following list is meant to serve as a repository for compiling a list of such ideas.
* The idea of the Pythagoreans that all numbers can be expressed as a ratio of two whole numbers. This was disproved by one of Pythagoras' own disciples, Hippasus, who showed that the square root of two is what we today call an irrational number. One story claims that he was thrown off the ship in which he and some other Pythagoreans were sailing because his discovery was too heretical.
* Euclid's parallel postulate stated that if two lines cross a third in a plane in such a way that the sum of the "interior angles" is not 180° then the two lines meet. Furthermore, he implicitly assumed that two separate intersecting lines meet at only one point. These assumptions were believed to be true for more than 2000 years, but in light of General Relativity at least the second can no longer be considered true. In fact the very notion of a straight line in four-dimensional curved space-time has to be redefined, which one can do as a geodesic. (But the notion of a plane does not carry over.) It is now recognized that Euclidean geometry can be studied as a mathematical abstraction, but that the universe is non-Euclidean.
* Fermat conjectured that all numbers of the form $2^{2^{m}}+1$ (known as Fermat numbers) were prime. However, this conjecture was disproved by Euler, who found that $2^{2^{5}}+1=4,294,967,297=641\times 6,700,417.$
* The idea that transcendental numbers were unusual. Disproved by Georg Cantor who showed that there are so many transcendental numbers that it is impossible to make a one-to-one mapping between them and the algebraic numbers. In other words, the cardinality of the set of transcendentals (denoted $\beth _{1}$) is greater than that of the set of algebraic numbers ($\aleph _{0}$).
* Bernhard Riemann, at the end of his famous 1859 paper "On the Number of Primes Less Than a Given Magnitude", stated (based on his results) that the logarithmic integral gives a somewhat too high estimate of the prime-counting function. The evidence also seemed to indicate this. However, in 1914 J. E. Littlewood proved that this was not always the case, and in fact it is now known that the first x for which $\pi (x)>\mathrm {li} (x)$ occurs somewhere before 10$^{317}$. See Skewes' number for more detail.
* Naïvely it might be expected that a continuous function must have a derivative or else that the set of points where it is not differentiable should be "small" in some sense. This was disproved in 1872 by Karl Weierstrass, and in fact examples had been found earlier of functions that were nowhere differentiable (see Weierstrass function). According to Weierstrass in his paper, earlier mathematicians including Gauss had often assumed that such functions did not exist.
* It was conjectured in 1919 by George Pólya, based on the evidence, that most numbers less than any particular limit have an odd number of prime factors. However, this Pólya conjecture was disproved in 1958. It turns out that for some values of the limit (such as values a bit more than 906 million), most numbers less than the limit have an even number of prime factors.
* Erik Christopher Zeeman tried for 7 years to prove that one cannot untie a knot on a 4-sphere. Then one day he decided to try to prove the opposite, and he succeeded in a few hours.
* A "theorem" of Jan-Erik Roos in 1961 stated that in an [AB4$^*$] abelian category, lim$^1$ vanishes on Mittag-Leffler sequences. This "theorem" was used by many people since then, but it was disproved by counterexample in 2002 by Amnon Neeman.