# abc conjecture

The abc conjecture (also known as the Oesterlé–Masser conjecture) is a conjecture in number theory, first proposed by Joseph Oesterlé (1988) and David Masser (1985). It is stated in terms of three positive integers, a, b and c (hence the name) that are relatively prime and satisfy a + b = c. If d denotes the product of the distinct prime factors of abc, the conjecture essentially states that d is usually not much smaller than c. In other words: if a and b are composed from large powers of primes, then c is usually not divisible by large powers of primes. The precise statement is given below. The abc conjecture originated as the outcome of attempts by Oesterlé and Masser to understand the Szpiro conjecture about elliptic curves.[1] The latter conjecture has more geometric structures involved in its statement in comparison with the abc conjecture.

Field Number theory Joseph OesterléDavid Masser 1985 Modified Szpiro conjecture

The abc conjecture and its versions express, in concentrated form, some fundamental feature of various problems in Diophantine geometry. A number of famous conjectures and theorems in number theory would follow immediately from the abc conjecture or its versions. Goldfeld (1996) described the abc conjecture as "the most important unsolved problem in Diophantine analysis".

Various attempts to prove the abc conjecture were made, but none have been accepted by the mainstream mathematical community so far and the conjecture is still largely recognized to be unproven as of 2019.

## Formulations

Before we state the conjecture we introduce the notion of the radical of an integer: for a positive integer n, the radical of n, denoted rad(n), is the product of the distinct prime factors of n. For example

If a, b, and c are coprime[notes 1] positive integers such that a + b = c, it turns out that "usually" c < rad(abc). The abc conjecture deals with the exceptions. Specifically, it states that:

ABC conjecture. For every positive real number ε, there exist only finitely many triples (a, b, c) of coprime positive integers, with a + b = c, such that
${\displaystyle c>\operatorname {rad} (abc)^{1+\varepsilon }.}$

An equivalent formulation:

ABC conjecture II. For every positive real number ε, there exists a constant Kε such that for all triples (a, b, c) of coprime positive integers, with a + b = c:
${\displaystyle c

A third equivalent formulation of the conjecture involves the quality q(a, b, c) of the triple (a, b, c), defined as

${\displaystyle q(a,b,c)={\frac {\log(c)}{\log {\big (}\operatorname {rad} (abc){\big )}}}.}$

For example:

q(4, 127, 131) = log(131) / log(rad(4·127·131)) = log(131) / log(2·127·131) = 0.46820...
q(3, 125, 128) = log(128) / log(rad(3·125·128)) = log(128) / log(30) = 1.426565...

A typical triple (a, b, c) of coprime positive integers with a + b = c will have c < rad(abc), i.e. q(a, b, c) < 1. Triples with q > 1 such as in the second example are rather special, they consist of numbers divisible by high powers of small prime numbers.

ABC conjecture III. For every positive real number ε, there exist only finitely many triples (a, b, c) of coprime positive integers with a + b = c such that q(a, b, c) > 1 + ε.

Whereas it is known that there are infinitely many triples (a, b, c) of coprime positive integers with a + b = c such that q(a, b, c) > 1, the conjecture predicts that only finitely many of those have q > 1.01 or q > 1.001 or even q > 1.0001, etc. In particular, if the conjecture is true, then there must exist a triple (a, b, c) that achieves the maximal possible quality q(a, b, c) .

## Examples of triples with small radical

The condition that ε > 0 is necessary as there exist infinitely many triples a, b, c with rad(abc) < c. For example, let

${\displaystyle a=1,\quad b=2^{6n}-1,\quad c=2^{6n},\qquad n>1.}$

The integer b is divisible by 9:

${\displaystyle b=2^{6n}-1=64^{n}-1=(64-1)(\cdots )=9\cdot 7\cdot (\cdots ).}$

Using this fact we calculate:

{\displaystyle {\begin{aligned}\operatorname {rad} (abc)&=\operatorname {rad} (a)\operatorname {rad} (b)\operatorname {rad} (c)\\&=\operatorname {rad} (1)\operatorname {rad} \left(2^{6n}-1\right)\operatorname {rad} \left(2^{6n}\right)\\&=2\operatorname {rad} \left(2^{6n}-1\right)\\&=2\operatorname {rad} \left(9\cdot {\tfrac {b}{9}}\right)\\&\leqslant 2\cdot 3\cdot {\tfrac {b}{9}}\\&=2{\tfrac {b}{3}}\\&<{\tfrac {2}{3}}c.\end{aligned}}}

By replacing the exponent 6n by other exponents forcing b to have larger square factors, the ratio between the radical and c can be made arbitrarily small. Specifically, let p > 2 be a prime and consider

${\displaystyle a=1,\quad b=2^{p(p-1)n}-1,\quad c=2^{p(p-1)n},\qquad n>1.}$

Now we claim that b is divisible by p2:

{\displaystyle {\begin{aligned}b&=2^{p(p-1)n}-1\\&=\left(2^{p(p-1)}\right)^{n}-1\\&=\left(2^{p(p-1)}-1\right)(\cdots )\\&=p^{2}\cdot r(\cdots ).\end{aligned}}}

The last step uses the fact that p2 divides 2p(p−1) − 1. This follows from Fermat's little theorem, which shows that, for p > 2, 2p−1 = pk + 1 for some integer k. Raising both sides to the power of p then shows that 2p(p−1) = p2(...) + 1.

And now with a similar calculation as above we have

${\displaystyle \operatorname {rad} (abc)<{\tfrac {2}{p}}c.}$

A list of the highest-quality triples (triples with a particularly small radical relative to c) is given below; the highest quality, 1.6299, was found by Eric Reyssat (Lando & Zvonkin 2004, p. 137) for

a = 2,
b = 310·109 = 6436341,
c = 235 = 6436343,

## Some consequences

The abc conjecture has a large number of consequences. These include both known results (some of which have been proven separately since the conjecture has been stated) and conjectures for which it gives a conditional proof. While an earlier proof of the conjecture would have been more significant in terms of consequences, the abc conjecture itself remains of interest for the other conjectures it would prove, together with its numerous links with deep questions in number theory. The consequences include:

• Roth's theorem on diophantine approximation of algebraic numbers.[2]
• The Mordell conjecture (already proven in general by Gerd Faltings).[3]
• As equivalent, Vojta's conjecture in dimension 1.[4]
• The Erdős–Woods conjecture allowing for a finite number of counterexamples.[5]
• The existence of infinitely many non-Wieferich primes in every base b > 1.[6]
• The weak form of Marshall Hall's conjecture on the separation between squares and cubes of integers.[7]
• The Fermat–Catalan conjecture, a generalization of Fermat's last theorem concerning powers that are sums of powers.[8]
• The L-function L(s, χd) formed with the Legendre symbol, has no Siegel zero, given a uniform version of the abc conjecture in number fields, not just the abc conjecture as formulated above for rational integers.[9]
• A polynomial P(x) has only finitely many perfect powers for all integers x if P has at least three simple zeros.[10]
• A generalization of Tijdeman's theorem concerning the number of solutions of ym = xn + k (Tijdeman's theorem answers the case k = 1), and Pillai's conjecture (1931) concerning the number of solutions of Aym = Bxn + k.
• As equivalent, the Granville–Langevin conjecture, that if f is a square-free binary form of degree n > 2, then for every real β > 2 there is a constant C(f, β) such that for all coprime integers x, y, the radical of f(x, y) exceeds C · max{|x|, |y|}nβ.[11]
• As equivalent, the modified Szpiro conjecture, which would yield a bound of rad(abc)1.2+ε.[12].
• Dąbrowski (1996) has shown that the abc conjecture implies that the Diophantine equation n! + A = k2 has only finitely many solutions for any given integer A.
• There are ~cfN positive integers nN for which f(n)/B' is square-free, with cf > 0 a positive constant defined as:[13]
${\displaystyle c_{f}=\prod _{{\text{prime }}p}x_{i}\left(1-{\frac {\omega \,\!_{f}(p)}{p^{2+q_{p}}}}\right).}$
• Fermat's Last Theorem has a famously difficult proof by Andrew Wiles. However it follows easily, at least for ${\displaystyle n\geq 6}$, from an effective form of a weak version of the abc conjecture. The abc conjecture says the lim sup of the set of all qualities (defined above) is 1, which implies the much weaker assertion that there is a finite upper bound for qualities. The conjecture that 2 is such an upper bound suffices for a very short proof of Fermat's Last Theorem for ${\displaystyle n\geq 6}$.[14]
• The Beal conjecture, a generalization of Fermat's last theorem proposing that if A, B, C, x, y, and z are positive integers with Ax + By = Cz and x, y, z > 2, then A, B, and C have a common prime factor. The abc conjecture would imply that there are only finitely many counterexamples.
• Lang’s conjecture, a lower bound for the height of a non-torsion rational point of an elliptic curve.

## Theoretical results

The abc conjecture implies that c can be bounded above by a near-linear function of the radical of abc. Bounds are known that are exponential. Specifically, the following bounds have been proven:

${\displaystyle c<\exp {\left(K_{1}\operatorname {rad} (abc)^{15}\right)}}$ (Stewart & Tijdeman 1986),
${\displaystyle c<\exp {\left(K_{2}\operatorname {rad} (abc)^{{\frac {2}{3}}+\varepsilon }\right)}}$ (Stewart & Yu 1991), and
${\displaystyle c<\exp {\left(K_{3}\operatorname {rad} (abc)^{{\frac {1}{3}}+\varepsilon }\right)}}$ (Stewart & Yu 2001).

In these bounds, K1 is a constant that does not depend on a, b, or c, and K2 and K3 are constants that depend on ε (in an effectively computable way) but not on a, b, or c. The bounds apply to any triple for which c > 2.

## Computational results

In 2006, the Mathematics Department of Leiden University in the Netherlands, together with the Dutch Kennislink science institute, launched the ABC@Home project, a grid computing system, which aims to discover additional triples a, b, c with rad(abc) < c. Although no finite set of examples or counterexamples can resolve the abc conjecture, it is hoped that patterns in the triples discovered by this project will lead to insights about the conjecture and about number theory more generally.

Distribution of triples with q > 1[15]
q
c
q > 1 q > 1.05 q > 1.1 q > 1.2 q > 1.3 q > 1.4
c < 102 644200
c < 103 311714831
c < 104 12074502283
c < 105 41824015251136
c < 106 1,2686673791022911
c < 107 3,4991,6698562106017
c < 108 8,9873,8691,8013849825
c < 109 22,3168,7423,69370614434
c < 1010 51,67718,2337,0351,15921851
c < 1011 116,97837,61213,2661,94732764
c < 1012 252,85673,71423,7733,02845574
c < 1013 528,275139,76241,4384,51959984
c < 1014 1,075,319258,16870,0476,66576998
c < 1015 2,131,671463,446115,0419,497998112
c < 1016 4,119,410812,499184,72713,1181,232126
c < 1017 7,801,3341,396,909290,96517,8901,530143
c < 1018 14,482,0652,352,105449,19424,0131,843160

As of May 2014, ABC@Home had found 23.8 million triples.[16]

Highest-quality triples[17]
Rank q a b c Discovered by
1 1.62992310·109235Eric Reyssat
2 1.626011232·56·73221·23Benne de Weger
3 1.623519·13077·292·31828·322·54Jerzy Browkin, Juliusz Brzezinski
4 1.5808283511·13228·38·173Jerzy Browkin, Juliusz Brzezinski, Abderrahmane Nitaj
5 1.567912·3754·7Benne de Weger

Note: the quality q(a, b, c) of the triple (a, b, c) is defined above.

The abc conjecture is an integer analogue of the Mason–Stothers theorem for polynomials.

A strengthening, proposed by Baker (1998), states that in the abc conjecture one can replace rad(abc) by

where ω is the total number of distinct primes dividing a, b and c.[18]

Andrew Granville noticed that the minimum of the function ${\displaystyle {\big (}\varepsilon ^{-\omega }\operatorname {rad} (abc){\big )}^{1+\varepsilon }}$ over ${\displaystyle \varepsilon >0}$ occurs when ${\displaystyle \varepsilon ={\frac {\omega }{\log {\big (}\operatorname {rad} (abc){\big )}}}.}$

This incited Baker (2004) to propose a sharper form of the abc conjecture, namely:

${\displaystyle c<\kappa \operatorname {rad} (abc){\frac {{\Big (}\log {\big (}\operatorname {rad} (abc){\big )}{\Big )}^{\omega }}{\omega !}}}$

with κ an absolute constant. After some computational experiments he found that a value of ${\displaystyle 6/5}$ was admissible for κ.

This version is called "explicit abc conjecture".

Baker (1998) also describes related conjectures of Andrew Granville that would give upper bounds on c of the form

${\displaystyle K^{\Omega (abc)}\operatorname {rad} (abc),}$

where Ω(n) is the total number of prime factors of n, and

${\displaystyle O{\big (}\operatorname {rad} (abc)\Theta (abc){\big )},}$

where Θ(n) is the number of integers up to n divisible only by primes dividing n.

Robert, Stewart & Tenenbaum (2014) proposed a more precise inequality based on Robert & Tenenbaum (2013). Let k = rad(abc). They conjectured there is a constant C1 such that

${\displaystyle c

holds whereas there is a constant C2 such that

${\displaystyle c>k\exp \left(4{\sqrt {\frac {3\log k}{\log \log k}}}\left(1+{\frac {\log \log \log k}{2\log \log k}}+{\frac {C_{2}}{\log \log k}}\right)\right)}$

holds infinitely often.

Browkin & Brzeziński (1994) formulated the n conjecture—a version of the abc conjecture involving n > 2 integers.

## Claimed proofs of abc

Lucien Szpiro proposed a solution in 2007, but it was found to be incorrect shortly afterwards.[19]

In August 2012 Shinichi Mochizuki claimed a proof of the abc conjecture.[20] He released a series of four preprints developing a new theory called Inter-universal Teichmüller theory (IUTT) which is then applied to prove several famous conjectures in number theory, including the abc conjecture but also Szpiro's conjecture, the hyperbolic Vojta's conjecture.[21] The papers have not been accepted by the mathematical community as providing a proof of abc.[22] This is not only because of their impenetrability,[23] but also because at least one specific point in the argument has been identified as a gap by some other experts.[24] Though a few mathematicians have vouched for the correctness of the proof,[25] and have attempted to communicate their understanding via workshops on inter-universal Teichmüller theory, they have failed to convince the number theory community at large.[26][27] In March 2018, Peter Scholze and Jakob Stix visited Kyoto for discussions with Mochizuki.[28][29] While they did not resolve the differences, they brought them into clearer focus. Scholze and Stix concluded that the gap was "so severe that … small modifications will not rescue the proof strategy";[30] Mochizuki claimed that they misunderstood vital aspects of the theory and made invalid simplifications.[31][32][33]

## Notes

1. When a + b = c, coprimeness of a, b, c implies pairwise coprimeness of a, b, c. So in this case, it does not matter which concept we use.

## Citations

1. Fesenko, Ivan (2015), "Arithmetic deformation theory via arithmetic fundamental groups and nonarchimedean theta functions, notes on the work of Shinichi Mochizuki" (PDF), European Journal of Mathematics, 1 (3): 405–440, doi:10.1007/s40879-015-0066-0.
2. The ABC-conjecture, Frits Beukers, ABC-DAY, Leiden, Utrecht University, 9 September 2005.
3. Mollin (2009); Mollin (2010, p. 297)
4. Granville, Andrew; Tucker, Thomas (2002). "It's As Easy As abc" (PDF). Notices of the AMS. 49 (10): 1224–1231.
5. "Synthese resultaten", RekenMeeMetABC.nl (in Dutch), archived from the original on December 22, 2008, retrieved October 3, 2012.
6. "Data collected sofar", ABC@Home, archived from the original on May 15, 2014, retrieved April 30, 2014
7. "100 unbeaten triples". Reken mee met ABC. 2010-11-07.
8. Bombieri & Gubler (2006), p. 404.
9. "Finiteness Theorems for Dynamical Systems", Lucien Szpiro, talk at Conference on L-functions and Automorphic Forms (on the occasion of Dorian Goldfeld's 60th Birthday), Columbia University, May 2007. See Woit, Peter (May 26, 2007), "Proof of the abc Conjecture?", Not Even Wrong.
10. Ball, Peter (10 September 2012). "Proof claimed for deep connection between primes". Nature. doi:10.1038/nature.2012.11378. Retrieved 19 March 2018.
11. Mochizuki, Shinichi (May 2015). Inter-universal Teichmuller Theory IV: Log-volume Computations and Set-theoretic Foundations, available at http://www.kurims.kyoto-u.ac.jp/~motizuki/papers-english.html
12. "The ABC conjecture has still not been proved". December 17, 2017. Retrieved March 17, 2018.
13. Revell, Timothy (September 7, 2017). "Baffling ABC maths proof now has impenetrable 300-page 'summary'". New Scientist.
14. "The ABC conjecture has still not been proved, comment by Bcnrd". December 22, 2017. Retrieved March 18, 2017.
15. Fesenko, Ivan. "Fukugen". Inference. Retrieved 19 March 2018.
16. Conrad, Brian (December 15, 2015). "Notes on the Oxford IUT workshop by Brian Conrad". Retrieved March 18, 2018.
17. Castelvecchi, Davide (8 October 2015). "The biggest mystery in mathematics: Shinichi Mochizuki and the impenetrable proof". Nature. 526 (7572): 178–181. Bibcode:2015Natur.526..178C. doi:10.1038/526178a.
18. Klarreich, Erica (September 20, 2018). "Titans of Mathematics Clash Over Epic Proof of ABC Conjecture". Quanta Magazine.
19. "March 2018 Discussions on IUTeich". Retrieved October 2, 2018. Web-page by Mochizuki describing discussions and linking consequent publications and supplementary material
20. Scholze, Peter; Stix, Jakob. "Why abc is still a conjecture" (PDF). Retrieved September 23, 2018. (updated version of their May report)
21. Mochizuki, Shinichi. "Report on Discussions, Held during the Period March 15 – 20, 2018, Concerning Inter-Universal Teichmüller Theory" (PDF). Retrieved February 1, 2019. the … discussions … constitute the first detailed, … substantive discussions concerning negative positions … IUTch.
22. Mochizuki, Shinichi. "Comments on the manuscript by Scholze-Stix concerning Inter-Universal Teichmüller Theory" (PDF). Retrieved October 2, 2018.
23. Mochizuki, Shinichi. "Comments on the manuscript (2018-08 version) by Scholze-Stix concerning Inter-Universal Teichmüller Theory" (PDF). Retrieved October 2, 2018.