# What are some correct results discovered with incorrect (or no) proofs?

Many famous results were discovered through non-rigorous proofs, with correct proofs being found only later and with greater difficulty. One that is well known is Euler's 1737 proof that

$1+\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\cdots =\frac{\pi^2}{6}$

in which he pretends that the power series for $\frac{\sin\sqrt{x}}{\sqrt{x}}$ is an infinite polynomial and factorizes it from knowledge of its roots.

Another example, of a different type, is the Jordan curve theorem. In this case, the theorem seems obvious, and Jordan gets credit for realizing that it requires proof. However, the proof was harder than he thought, and the first rigorous proof was found some decades later than Jordan's attempt. Many of the basic theorems of topology are like this.

Then of course there is Ramanujan, who is in a class of his own when it comes to discovering theorems without proving them.

I'd be interested to see other examples, and in your thoughts on what the examples reveal about the connection between discovery and proof.

Clarification. When I posed the question I was hoping for some explanations for the gap between discovery and proof to emerge, without any hinting from me. Since this hasn't happened much yet, let me suggest some possible explanations that I had in mind:

Physical intuition. This lies behind results such as the Jordan curve theorem, Riemann mapping theorem, Fourier analysis.

Lack of foundations. This accounts for the late arrival of rigor in calculus, topology, and (?) algebraic geometry.

Complexity. Hard results cannot proved correctly the first time, only via a series of partially correct, or incomplete, proofs. Example: Fermat's last theorem.

I hope this gives a better idea of what I was looking for. Feel free to edit your answers if you have anything to add.

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I was thinking also of stuff like Witten. –  Steve Huntsman Jun 11 '10 at 1:01
In Tom Hales account of Jordan's proof, he states that there is essentially no problem with Jordan's original proof, and that claims to the contrary are themselves wrong or based on misunderstandings. As far as I can tell, he is correct, and there is no reason to impugn Jordan's original proof. (See "Jordan's proof of the Jordan curve theorem" at math.pitt.edu/~thales/papers ) –  Emerton Jun 11 '10 at 2:49
@Emerton. I stand corrected. Maybe Jordan's proof should be in the same category as Heegner's: thought to be incorrect, but essentially correct when properly understood. –  John Stillwell Jun 11 '10 at 3:09
A further remark: I think that is important to distinguish between polishing an argument, or perhaps interpreting it in terms of contemporary language and formalism, which will almost always be required when reading arguments (especially subtle ones) from 100 or more years ago, and genuinely incomplete arguments. As an example of the latter, one can think of Riemann's arguments with the Dirichlet principle, where this result was simply taken as an axiom. Additional work was genuinely required to validate the Dirichlet principle, and thus complete Riemann's arguments. –  Emerton Jun 11 '10 at 5:59
I would argue that (although it came after the drive for rigor had already started thanks to Cantor, Weierstrass, et al.) the dawn of modern statistical and quantum physics had a great deal to do with the consolidation of rigor throughout mathematics. Indeed, ergodic theory and functional analysis owe a great deal to these disciplines, and neither could have existed in the time of (say) Euler because the approach to mathematics was different. –  Steve Huntsman Jun 11 '10 at 12:32

In 1905, Lebesgue gave a "proof" of the following theorem:

If $f:\mathbb{R}^2\to\mathbb{R}$ is a Baire function such that for every x, there is a unique y such that f(x,y)=0, then the thus implicitly defined function is Baire.

He made use of the "trivial fact" that the projection of a Borel set is a Borel set. This turns out to be wrong, but the result is still true. Souslin spotted the mistake, and named continuous images of Borel sets analytic sets. So a mistake of Lebesgue led to the rich theory of analytic sets. Lebesgue seemingly enjoyed this fact and mentioned it in the foreword to a book of Souslins's teacher Lusin.

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This is an interesting category of incorrect proof: where the mistake is actually fruitful. I'd like to see more of these! –  John Stillwell Jun 11 '10 at 5:57
–  Qiaochu Yuan Jun 11 '10 at 8:36

This identity is still not proven:

$$\sum_{n=0}^\infty \left(\frac{1}{(7n+1)^2}+\frac{1}{(7n+2)^2}-\frac{1}{(7n+3)^2}+\frac{1}{(7n+4)^2}-\frac{1}{(7n+5)^2}-\frac{1}{(7n+6)^2}\right)=\frac{24}{7\sqrt{7}}\int_{\pi/3}^{\pi/2} \log \left| \frac{\tan t + \sqrt{7} }{\tan t - \sqrt{7} } \right| dt$$

It arose from physical applications.

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Very interesting. Any references? –  Andrey Rekalo Nov 5 '10 at 8:14
From here: crd.lbl.gov/~dhbailey/dhbpapers/math-future.pdf It has been verified up to 20,000 digits. –  Anixx Nov 5 '10 at 8:23
For what it's worth, the left hand side is the value of the Dirichlet L-series for the nontrivial character with conductor $7$ at $s = 2$. –  Franz Lemmermeyer Nov 5 '10 at 14:03
The left hand side is also the Legendre symbol $\displaystyle \biggl(\frac{n}{7}\biggr)$ –  Chandrasekhar Jul 7 '12 at 7:36
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Heegner's proof in 1952 that there is no tenth imaginary quadratic field of class number one is an interesting example. It was thought to be incorrect because of some gaps. Stark gave a correct proof in 1967 and explained how it was essentially the same as Heegner's proof. In 1969 Stark formally filled in the gap in Heegner's proof. Heegner "died before anyone really understood what he had done" (Goldfeld). This information comes from http://en.wikipedia.org/wiki/Stark-Heegner_theorem.

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The Nielsen realization problem. Let $S$ be a compact oriented topological surface and let $\text{Mod}(S)$ be its mapping class group, ie the group of orientation preserving diffeomorphisms of $S$ modulo isotopy. There is a natural surjection $\text{Diff}^+(S) \rightarrow \text{Mod}(S)$. The Nielsen realization problem was the conjecture (due to Jacob Nielsen) that every finite subgroup of $\text{Mod}(S)$ can be lifted to a finite subgroup of $\text{Diff}^+(S)$ (and thus is a subgroup of the group of automorphisms of a Riemann surface).

Nielsen proved this for finite cyclic subgroups (this is very nontrivial!), and a number of other people slowly chipped away at other classes of finite subgroups. In 1959, Kravetz published a paper which purported to prove that Teichmuller space is negatively curved. A "center of mass" argument would then establish that every finite subgroup of $\text{Mod}(S)$ fixes a point in Teichmuller space, and it then follows easily that the finite subgroup can be lifted to $\text{Diff}^+(S)$.

This was an important result, and Kravetz's paper was frequently quoted. However, in 1971 Linch pointed out in his thesis that Kravetz's paper had an error! In fact, in his 1974 thesis Howie Masur proved that Teichmuller space is not negatively curved (in a pretty strong sense).

Finally, in 1980 Steve Kerckhoff proved that Teichmuller space, while not negatively curved, did satisfy a subtle negative-curvature like property which gave the desired result.

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Wow, that's a pretty sobering story. Almost a poster child for Lamport's thesis that much of the mathematical literature is shot through with errors in proofs. –  Todd Trimble Aug 27 '11 at 18:20

The "Yamabe problem": Every compact Riemannian manifold admits a conformally-related metric with constant scalar curvature. Yamabe thought he had proved this in 1960, but his proof had--I'm not making this up--a sign error. The error was discovered by Neil Trudinger in 1968, after Yamabe's death. As I understand it, Trudinger was working on a similar nonlinear elliptic PDE problem (with a critical Sobolev exponent) and got stuck, so he looked at Yamabe's paper to see how Yamabe had dealt with the same issue. Turned out he hadn't. Trudinger was able to give a partial solution to the problem; later Aubin expanded it to cover more cases, and finally in 1984 Rick Schoen was able to prove it the cases that Aubin had left open (with a small gap in the higher-dimensional case that was repaired by Schoen and Yau in 1988). The proof surprisingly used the positive mass theorem from general relativity.

Yamabe's original paper never attracted much attention until the error was found. But because of the subtlety of the methods required to fill in the gap, it has become a model for applications of nonlinear elliptic PDE to geometry, especially to conformally invariant problems and other problems with critical regularity.

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Interesting! I had heard of this theorem before, but not of the error, or that it used the positive mass theorem. Does this mean there's some non-physics geometrical insight behind the positive mass theorem? I had always thought of this as being true as a consequence of the dominated energy condition, which is a very "physical" condition to require. –  jeremy Jun 12 '10 at 2:26
Also, I wouldn't say that "Schoen was able to prove the whole theorem". Aubin proved it for all dimensions $\geq 6$ when $M$ is not locally conformally flat, and Schoen proved it for 3,4, and 5 and all locally conformally flat manifolds. In fact, it is a curious fact that Schoen's proof doesn't work in the cases where Aubin's worked. (1 dimension has no curvature, and 2 dimension follows from uniformization theorem.) –  Willie Wong Jul 8 '10 at 21:39
A very readable account of the history of the Yamabe problem is available ams.org/journals/bull/1987-17-01/S0273-0979-1987-15514-5 –  Willie Wong Jul 8 '10 at 21:45

There are at least two Hilbert problems that were considered to be solved, but the proofs turned out to be incomplete, as pointed out by Yulii Ilyashenko.

1. In 1923 Dulac published a 140+ page memoir purporting to show that a polynomial vector field on the plane has only finitely many limit cycles, the second part of the 16th Hilbert problem. The memoir was difficult to read, but the claim was generally accepted until in 1981 Ilyashenko found a serious gap. Full proofs were obtained independently by Écalle and Ilyashenko around 1991. Read the full story.

2. Existence of linear differential equations having a prescribed monodromic group was the subject of the 21st Hilbert problem, also known as the Riemann-Hilbert problem. From Wikipedia article:

Josip Plemelj published a solution in 1908. This work was for a long time accepted as a definitive solution; there was work of G. D. Birkhoff in 1913 also, but the whole area, including work of Ludwig Schlesinger on isomonodromic deformations that would much later be revived in connection with soliton theory, went out of fashion. Plemelj produced a 1964 monograph Problems in the Sense of Riemann and Klein, (Pure and Applied Mathematics, no. 16, Interscience Publishers, New York) summing up his work. A few years later the Soviet mathematician Yuliy S. Il'yashenko and others started raising doubts about Plemelj's work. In fact, Plemelj correctly proves that any monodromy group can be realised by a regular linear system which is Fuchsian at all but one of the singular points. Plemelj's claim that the system can be made Fuchsian at the last point as well is wrong. (Il'yashenko has shown that if one of the monodromy operators is diagonalizable, then Plemelj's claim is true.)
Indeed in 1989 Soviet mathematician Andrey A. Bolibrukh (1950–2003) found a counterexample to Plemelj's statement. This is commonly viewed as providing a counterexample to the precise question Hilbert had in mind; Bolibrukh showed that for a given pole configuration certain monodromy groups can be realised by regular, but not by Fuchsian systems.
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I was just starting to wonder about wrong statements with wrong proofs that everyone believed for a long time. Nice examples! –  Paul Siegel Jun 11 '10 at 12:18

Riemann's flawed proof of the Riemann mapping theorem which crucially relied on Dirichlet's principle.

The theorem was stated by Bernhard Riemann in 1851 in his PhD thesis. Lars Ahlfors wrote once, concerning the original formulation of the theorem, that it was “ultimately formulated in terms which would defy any attempt of proof, even with modern methods”. Riemann's flawed proof depended on the Dirichlet principle (whose name was created by Riemann himself), which was considered sound at the time. However, Karl Weierstraß found that this principle was not universally valid. Later, David Hilbert was able to prove that, to a large extent, the Dirichlet principle is valid under the hypothesis that Riemann was working with.

The first proof of the theorem is due to Constantin Carathéodory, who published it in 1912. His proof used Riemann surfaces and it was simplified by Paul Koebe two years later in a way which did not require them.

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Another one we can credit to physical intuition. –  John Stillwell Jun 11 '10 at 0:47
The earliest known correct proof of Riemann mapping theorem appears in a paper of William Osgood in 1900.It is in volume 1 of the transactions of the AMS. –  Mohan Ramachandran Jun 11 '10 at 16:32
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The four-color theorem.

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Is this long list of two word answers OK? This almost feels like some sort of spam. –  Adrián Barquero Jun 11 '10 at 2:39
It's a big list, proper form is to separate them. –  Steve Huntsman Jun 11 '10 at 3:06
This is an excellent answer. In the 19th century Heawood proved the 5 color theorem and gave a false proof of the 4 color theorem. But his ideas in the proof of the 5 color theorem were the basic starting point for all further progress. –  paul Monsky Jun 11 '10 at 10:42
Indeed Heawood proved the 5-color theorem. But I'm not aware that he gave an incorrect proof of the 4-color theorem. What he is known for doing is finding a flaw in an 1879 supposed proof, by Kempe, that had stood for 11 years. Perhaps at least as impressive, he determined the "Heawood number" -- an upper bound for the chromatic number -- for every compact surface, and conjectured it was the actual chromatic number. This number turned out to be the actual chromatic number of every compact surface except the Klein bottle, as shown by Ringel & Youngs (except for the sphere) in 1968. –  Daniel Asimov Jun 12 '10 at 2:12
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When Stephen Smale was a graduate student, he thought he had a proof of the Poincaré Conjecture as follows: Take a compact simply-connected 3-manifold M and remove the interiors of two disjoint 3-balls to get, say, M1 having as boundary two copies of S2. It is easy to show that M1 has a nonsingular vector field entering along one S2 and exiting along the other. Clearly by the simply-connectedness of M, each orbit entering on one boundary component must exit on the other one. Thus M1 must be S2 x [0,1] and hence by replacing the removed 3-balls, M must have been S3. QED.

I'm not sure who first pointed out the error, but undoubtedly understanding examples like this helped him appreciate the subtlety of the problem and ultimately prove the Generalized Poincaré Conjecture for dimensions ≥ 5.

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Daniel, could you please explain the error in the reasoning? –  Tom LaGatta Jun 11 '10 at 21:24
Sure. The sentence starting with "Clearly" isn't. In fact there exist orientable 1-foliations (which result from C<sup>1</sup> nonsingular vector fields as the solutions to the corresponding ODE) on even S<sup>2</sup> x [0,1] that are entering on one boundary component and exiting on the other, without every trajectory that enters on one boundary component exiting on the other one. –  Daniel Asimov Jun 12 '10 at 0:30
(cont'd) This can be achieved by starting with the canonical flow on S<sup>2</sup> x [0,1] (i.e., the one parallel to [0,1]) and introducing a "plug" -- a copy of S<sup>1</sup> x [0,1] x[0,1] -- on which the flow is altered. See, for instance, Plugging Flows by Percell and Wilson. For those with access, at < jstor.org/stable/pdfplus/1997824.pdf >. –  Daniel Asimov Jun 12 '10 at 0:34
I heard Smale tell a version of this story at the Clay conference in Paris a couple of months ago. He got interested in the Poincaré conjecture and spent a night coming up with a simple proof. The next morning he went to his advisor and explained the details, and all the time his advisor just sat there silent and nodded from time to time. Smale left the meeting a little frustrated that his proof hadn't been met with more interest, until he realized later that day that he had never used the hypothesis of simple connectedness. But yeah, he did say that this helped him in the proof for n>=5. –  Gunnar Magnusson Jul 9 '10 at 9:47

According to Weierstrass, Riemann knew about the existence of continuous nowhere differentiable functions. (Weierstrass' celebrated example was published in 1872, some 6 years after Riemann's death.) In his lectures, Riemann allegedly suggested the example $$f(x)=\sum\limits_{k=1}^{\infty}\frac{\sin k^2x}{k^2}$$ as early as 1861. He gave no proof and just mentioned that it could had been obtained from the theory of elliptic functions (see the historical note "Riemann’s example of a continuous “nondifferentiable” function continued" by S.L. Segal).

Hardy proved in 1916 that $f$ has no finite derivative at any $x=\pi\xi$ where $\xi$ is irrational but left the general case open.

It was only in 1970 that J. Gerver finally proved that the Riemann function is in fact differentiable when $$x=\pi\frac{2m+1}{2n+1},\qquad m,n\in\mathbb Z,$$ and $f'(x)=-1/2$ at these points ("The Differentiability of the Riemann Function at Certain Rational Multiples of π", ).

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The fundamental theorem of algebra was given incomplete proofs by d'Alembert, Euler, Lagrange, Laplace, Gauss.. http://en.wikipedia.org/wiki/Fundamental_theorem_of_algebra

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I suppose we can cite here Fermat's Last Theorem as a prime example, although I'm not really sure about the connection between discovery and proof here.

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You beat me to it,Adrian. Andrew Wiles original proof of the Taniyama-Shimura conjecture-the major result in modular form theory that has Fermat's famous theorum as a corollary-had a massive gap in it when it was first presented in oral lectures in 1993. With the help of his student Richard Taylor,the final,correct proof was published in 1995. All this is of course common knowledge-what I think a lot of mathematicians sort of forget in all this is the enormous path of discovery leading to Wiles' result. In many ways,it was the culmination of nearly a hundred years of progress. –  Andrew L Jun 11 '10 at 1:23
I think Adrian is referring to Fermat's discovery. –  Qiaochu Yuan Jun 11 '10 at 1:52
@Andrew: Quibble: Wiles did not prove (nor did he think or claim he did) the full T-S conjecture; he only established the 'semi-stable case', which was sufficient (from Ribet's work) to establish FLT. The proof of the full conjecture came later, taking off from the work of Wiles and Taylor. –  Arturo Magidin Jun 11 '10 at 2:47
Dear Andrew L, I think that to say there was a "massive gap" is not quite correct. There was a gap (and one could say that a miss is as good, or in this case, as bad, as a mile), but it was filled within a year or so, in his joint work with Taylor, and the fundamental structure, as well as many of the details, of the argument remained unchanged. In any event, this is no sense an instance of the situation John is envisaging in his question. –  Emerton Jun 11 '10 at 2:53
Adrián, let's not forget that Fermat also claimed many facts that aren't true. Fermat primes come to mind first. –  Victor Protsak Jun 11 '10 at 7:48

I submit for your consideration Euclid's fifth postulate. Given the amount of effort taken by people to prove it from his four other postulates, I consider it canny or lucky that Euclid chose to keep it as an axiom. Of course, it took unusual thinking and some discovery to realize that Euclid's fifth was indeed independent of the other postulates.

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Dehn's lemma was given an incorrect proof by Dehn in 1910; only in 1956 was a true proof found

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I suggest renaming it "Dehn's lemon". –  Victor Protsak Jun 11 '10 at 7:50
The proof was found by C. Papakyriakopoulos, right? It seems only fair to mention his name! –  Pete L. Clark Jun 22 '10 at 14:26
That's right, and I should have mentioned him. People with names longer than Nakayama's probably have a hard time getting things named after them. –  paul Monsky Jun 22 '10 at 22:30

How about mirror symmetry of Calabi-Yaus? This started from the observation by physicists that string theory on certain pairs of Calabi-Yaus gave identical theories. This has lead to a lot of work by physicists and mathematicians to understand what's going on, leading to things like the SYZ conjecture, homological mirror symmetry, etc.

So, more specifically physicists theories treat spacetime $M$ as something that locally looks like $M=\mathbb{R}^4\times X$ in such a way that $X$ is "small" by saying (roughly) operators (which represent observables) when "looking at things" below a certain energy scale can't see directly the dynamics associated with $X$. Associated with $M$ is a special kind of quantum field theory called a superconformal field theory (SCFT), which requires that $X$ be a Calabi-Yau 3-fold.

Various topological invariants of $X$ can tell us about how the SCFT behaves.

But it was discovered that the associated SCFTs don't uniquely determine $X$. It turns out there are pairs of Calabi-Yau 3-folds $(X,\hat{X})$ (called mirrors) that give the same SCFT.

From the SCFT point of view, these two mirror manifolds are related by an automorphism of the SCFT, which does not correspond to an automorphism of the Calabi-Yau manifold, but instead gives a mirror manifold in a way that switches cohomology groups around. It can also be thought of as switching complex structures with symplectic ones somehow.

From the rigorous point of view, though, not much of this is well-defined. It relies on the machinery of QFTs which no one has been able to come close to defining axiomatically, as well as string theory which relies on a lot of machinery that has the same kinds of problems.

Out of this came a number of more mathematically precise conjectures, such as the SYZ conjecture, which explains this in terms of special Lagrangian manifolds and fibrations of the mirror manifolds into it.

This also started ideas of homological mirror symmetry, which tries to describe this in terms of homology and derived categories.

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Euler "proved" that $\sum \mu(n)/n = 0$ by observing that $\sum \mu(n) n^{-s} = 1/\zeta(s)$ and setting $s = 1$. Actually, the result $\sum \mu(n)/n = 0$ was later proved by von Mangoldt, and shown to be equivalent to the prime number theorem by Landau.

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There are two famous examples from enumerative algebraic geometry. The Schubert calculus was used by Schubert to solve many elaborate enumerative problems, but it was only fairly recently that these results were verified according to modern standards of mathematical rigor. Also, string theory predicted some enumerative results that the mathematicians were only later able to verify.

More generally, modern theoretical physics has produced enormous numbers of mathematical results whose derivations are non-rigorous. Some of these have been rigorously verified but some remain open problems.

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The classification of finite simple groups was announced 1983 when Geoff Mason was still working on the quasithin case. I've heard somewhere that he lost his motivation then and never finished his 600+ pages manuscript. The gap was closed 20 years later by Michael Aschbacher and Steve Smith.

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Man, those finite group theorists of the 80's were hard-core, with all of those several-hundred-page papers of closely reasoned mathematics! –  Todd Trimble Apr 4 '11 at 15:13
@Jonathan: In 1983 all 26 sporadic groups were known and their existence and uniqueness proven (The "Atlas of Finite Groups" was published 1985). You could only complain that some proofs still were computer-assisted. –  Someone Apr 5 '11 at 7:24
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Another classic example is the Littlewood-Richardson rule for decomposing products of Schur polynomials. It was discovered and proved in some special cases in 1934 by Littlewood and Richardson. In 1938 Richardson published a purported proof which had some gaps; however, apparently he managed to write so obscurely that the result was accept at least until the '50's. The first complete proofs were found in the '70's by Schützenberger and Thomas.

This is definitely an example in which the trouble arose from the difficulty of the result, which involves from pretty thorny combinatorics. In his paper "The representation theory of the symmetric groups", Gordon James said the following : "Unfortunately the Littlewood–Richardson rule is much harder to prove than was at first suspected. The author was once told that the Littlewood–Richardson rule helped to get men on the moon but was not proved until after they got there."

Remark : The above chronology is taken from wikipedia. I learned the Littlewood-Richardson rule from modern accounts, but I have to admit that I've never tried to go back and read the early papers on the subject.

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In 1983 or 84, Frey announced that he could prove that Taniyama-Weil conjecture implies Fermat's last theorem. The proof was flawed but this announcement had spectacular consequences:

$\bullet$ Serre pulled out an unpublished conjecture of his and strengthened it so that Taniyama-Weil + $\varepsilon$ would imply FLT,

$\bullet$ Ribet proved enough of $\varepsilon$ so ensure that TW would imply FLT,

$\bullet$ Wiles realized that FLT would be proved as TW could not be ignored and so decided that it had to be by him (in doing so, he completely changed the way people thought about the field and this has led to impressive results including the proof of TW or of Sato-Tate conjecture),

$\bullet$ Shimura decided that he wanted his name attached to the conjecture and Lang made a campaign to remove Weil's...

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There is a more elementary, yet mathematically important and challenging "renormalization": the procedure by which Feigenbaum universality is proved (and its variants). I suggest that you expand your answer. –  Victor Protsak Jun 11 '10 at 2:45
I agree that an expansion of this answer would be illuminating, but I would not go so far as to pile on negative votes. –  j.c. Jun 11 '10 at 12:11
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In a sense, the entire field of ergodic theory was born from Boltzmann's incomplete proof of the H-theorem.

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Grunwald's Theorem on the existence of extensions satisfying local data was well known and widely used; Whaples even gave a second proof of this result before Wang found a counterexample and closed the gap. A similar mistake occurred when Shafarevich proved that solvable groups are Galois groups over the rationals - the case of 2-groups was "problematic".

On a more fundamental level, Kummer's proofs of unique factorization into prime ideal numbers had gaps because he did not know about the concept of integral closure. This gap was noticed and closed only by Dedekind.

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Just to complement Gerhard Paseman's answer. The story of how Girolamo Saccheri in early 1700's "almost" discovered hyperbolic geometry is quite amusing. Actually he died thinking he had proved the fifth postulate, but his argument was weak: "the hypothesis of the acute angle is absolutely false; because it is repugnant to the nature of straight lines". The sentence referes to his construction of a quadrilateral with two sides of equal length perpendicular to a given one. The acute angles are the ones opposite to the right ones. But Wikipedia explains this too...

In this example an ideological bias prevented the discovery of beautiful mathematics... I wonder if this still happens now a days, probably yes.

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The Kronecker-Weber theorem needed 3 proofs spanned upon 30 years before being completely proved (it states that all abelian extensions of ${\mathbb Q}$ can be found inside cyclotomic fields). It lead to class field theory.

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I guess the historically first example is the Theorem of Pythagoras, already known to the Babylonians but probably not discovered by a "proof" satisfying modern standards.

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According to M. Meo, Cauchy's proof of Cauchy's theorem (existence of elements of order a given prime p in every finite group of order divisible by a p) is wrong.

Cauchy works with subgroups of $S_n$, and his proof depends on the construction of what we now call a Sylow subgroup of $S_n$. This subgroup is obtained as a semidirect product, which Cauchy seems to say is actually a direct product (which would be abelian). I am not completely sure whether Cauchy was really wrong, or he did know what was going on, and simply lacked the appropriate language. In any case, would be an example of Lack of foundations.

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Looman (1923) proved that existence of partial derivatives of a function defined on an open subset of the complex plane is a sufficient condition for the function to be analytic. His proof had a gap that was fixed by Menchoff (1936) and we now have the Looman-Menchoff theorem.

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