# I would like to ask about examples where experimentation by computers have led to major mathematical advances.

## A new look

Now as the question is five years old and there are certainly more examples of mathematical advances via computer experimentation of various kinds, I propose to consider contributing new answers to the question.

## Motivation

I am aware about a few such cases and I think it will be useful to gather such examples together. I am partially motivated by the recent polymath5 which, at this stage, have become an interesting experimental mathematics project. So I am especially interested in examples of successful "mathematical data mining"; and cases which are close in spirit to the experimental nature of polymath5. My experience is that it can be, at times, very difficult to draw useful insights from computer data.

## Summary of answers according to categories

1) Mathematical conjectures or large body of work arrived at by examining experimental data - Classic

2) Mathematical conjectures or large body of work arrived at by examining experimental data - Current

"Maeda conjecture"; the work of Candès and Tao on compressed sensing; Certain Hankel determinants; Weari-Phelan structure; the connection of multiple zeta values to renormalized Feynman integrals; Thistlethwaite's discovery of links with trivial Jones polynomial; The Monstrous Moonshine; (NEW:) McKay's account on experimentation leading to mysterious "numerology" regarding the monster. (link to the answer); Haiman conjectures on the quotient ring by diagonal invariants

3) Computer-assisted proofs of mathematical theorems

Kepler's conjecture ; a new way to tile the plane with a pentagon: advances regarding bounded gaps between primes following Zhang's proof; Cartwright and Steger's work on fake projective planes; the Seifert-Weber dodecahedral space is not Haken; the four color theorem, the proof of the nonexistence of a projective plane of order 10; Knuth's work on a class of projective planes; The search for Mersenne primes; Rich Schwartz's work; The computations done by the 'Atlas of Lie groups' software of Adams, Vogan, du Cloux and many other; Cohn-Kumar proof for the densest lattice pacing in 24-dim; Kelvin's conjecture;

4) Computer programs that interactively or automatically lead to mathematical conjectures.

Graffiti

5) Various computer programs which allow proving automatically theorems or generating automatically proofs in a specialized field.

Wilf-Zeilberger formalism and software; FLAGTOOLS

6) Computer programs (both general purpose and special purpose) for verification of mathematical proofs.

The verification of a proof of Kepler's conjecture.

7) Large databases and other tools

Sloane's online encyclopedia for integers sequences; the inverse symbolic calculator.

8) Resources:

Journal of experimental mathematics; Herb Wilf's article: Mathematics, an experimental science in the Princeton Companion to Mathematics, genetic programming applications a fairly comprehensive website experimentalmath.info ; discovery and experimentation in number theory; Doron Zeilberger's classes called "experimental mathematics":math.rutgers.edu/~zeilberg/teaching.html; V.I. Arnol'd's two books on the subject of experimental mathematics in Russian, Experimental mathematics, Fazis, Moscow, 2005, and Experimental observation of mathematical facts, MCCME, Moscow, 2006

Answers with general look on experimental mathematics:

Computer experiments allow new avenues for productive strengthening of a problem (A category of experimental mathematics).

### Bounty:

There were many excellent answers so let's give the bounty to Gauss...

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en.wikipedia.org/wiki/… – Steve Huntsman Jan 17 '10 at 8:35
You might want to email Doron Zeilberger at Rutgers. – Charles Siegel Jan 17 '10 at 15:39
Indeed, Doron Zeilberger has even taught classes called "experimental mathematics": math.rutgers.edu/~zeilberg/teaching.html . – Qiaochu Yuan Jan 17 '10 at 15:50
@Mark: URLs are automatically turned into links in comments; no need for html markup. Corrected version of the link: emis.de/journals/EM – Anton Geraschenko Jan 20 '10 at 18:53
Doron has recently posted a "videoed lecture" about experimental mathematics: math.rutgers.edu/~zeilberg/mamarim/mamarimhtml/future.html – Douglas S. Stones Apr 3 '10 at 0:48

The Prime Number Theorem was conjectured by Gauss from looking (very hard, one can presume...) at a table of the primes $\leq10^6$. It is not with too much effort that one can read his Disquisitiones as a set of tricks to determine primality with as little work as possible, and one can understand the motivation: he was his own computer, in a way :P

(I don't know where Legendre got the statement from, but he must surely have had tables of primes too!)

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This may be apocryphal, but I remember being told that Gauss also conjectured and proved quadratic reciprocity after expanding 1/p in base q for various pairs of primes p, q. – Qiaochu Yuan Jan 18 '10 at 15:58
Considering Euler died when Gauss was seven or eight, I'd think that in some sense it's very likely Euler predated Gauss. (I don't know if Euler published his conjectured statement, though.) – Harrison Brown Jan 18 '10 at 16:29
You get 1/p as a/(q^n-1) so p is a factor of q^n-1, and the multiplicative order of q is n mod p. If (p-1)/n is even, q is a quadratic residue. – Douglas Zare Jan 18 '10 at 21:28
I would like to mention two "computers" of the 18th century. The first one is Anton Felkel who produced a table of primes in 1776. In scs.uiuc.edu/~mainzv/exhibitmath/exhibit/felkel.htm one can read that the Austrian artillery used his output in battles : just the paper, not numbers! In 1849 Gauss mentions in his 4 page letter to his student JF Encke that he used Vega's tables to confirm his estimate. Jurij Vega published his table of primes in 1797. Who knows if Felkel's tables were used for cartridges in 1789 when Vega commanded a battery of mortars at the battle for Belgrade. – Tomaž Pisanski Mar 11 '10 at 17:09

First, a brief story, then the punchline.

Elsewhere I mention a series of 0-1 matrices used for a combinatorial matrix result. This came about because of an aspiring undergraduate (Roger House) who was using a computer to search for 0-1 matrices of certain orders and determinant values.

He showed me a printout where he listed the first representative of each class (major key order, minor key absolute determinant value). He chose a lexicographical ordering of class members to pick this representative: this choice revealed a set of matrices with few differing entries, and those in just the first two or three columns. I would not blame someone else for saying it was a pattern screaming to be found.

Many a time since then, I think about how to organize computer-generated data (specifically, pick an explanatory set of representatives) that reveal patterns and structure. I think that experimental mathematics should not just include results, or even data analysis techniques, but heuristics on "how to make choices" so that patterns of note jump to the human eye. In the case above, lexicographic order was a choice made by one person that revealed a pattern of similarity to another person.

Gerhard "At Least, Raise Their Voices" Paseman, 2015.10.12

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(Answer by John Mckay:) That the product of two conjugate (type 2A) involutions of the monster, M lies in 9 conjugacy classes with order given by the coefficients of the highest root in affine E8, namely {1,2,3,4,5,6,4,2,3} -- and similarly for E7 and E6 with 2.Baby and 3.F24' respectively -- was experimental with very little evidence. See my note in Nature 305, 672, 20th Oct 1983 on numerology.

(GK: Indeed this is a very important example in the history of mathematics. Here is the Nature's paper)

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In a remarkable article from 1989, "Self-similarity and hairiness in the Mandelbrot set", Milnor formulates a number of conjectures (related to Feigenbaum universality). Citing Lisa Goldberg and Tony Phillips in the preface to Topological Methods in Modern Mathematics (Milnor's 60th birthday Festschrift, 1993),

OF special note is an unconventional article entitled Self-similarity and hairiness in the Mandelbrot set, where he presents some of his numerical experiments in holomorphic dynamics and uses them as evidence for a set of conjectures. ... The resulting data are so compelling as to suggest not only what the conjectures should say, but how they can be proved.

Lyubich later proved some of these conjectures in a groundbreaking article (Feigenbaum-Coullet-Tresser universality and Milnor's Hairiness conjecture, Annals of Mathematics, 1999), which also, for the first time, gave a conceptual proof of the Feigenbaum scaling law.

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Thurston's geometrization program began with Bob Riley's computer experiments - Thurston's intuition was actually initially the opposite (he did not believe that the figure eight knot could be hyperbolic until he saw the experimental result).

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Graffiti is a computer program which makes conjectures in various subfields of mathematics, particularly graph theory. (And also in chemistry.) It was written by written by Siemion Fajtlowicz. Here is a 1989 article about it.

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The solution of the Kepler's conjecture by Hales deserves a special answer since it gives an important example both for a proof to a mathematical theorem massively using computers, and 15 years later by an impressive project for demonstrating a formal proof for the conjecture that could be automatically verified.

The problem and the first project which in part was carried out by Hales with Ferguson is described in the Notices AMS paper "Cannonballs and Honeycombs" Another reference is a 2002 ICM survey A Computer Verification of the Kepler Conjecture.

The published account of the second project - The Flyspeck project can be found in the paper A formal proof of the Kepler conjecture.

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The re-launching of this question is quite timely, as experimental math was behind a beautiful recent discovery of a new way to tile the plane with a pentagon. Previously only 14 such tilings were known. The new and 15th tiling announced a few weeks ago was discovered by Casey Mann, Jennifer McLoud and David Von Derau using a computer search:

https://en.wikipedia.org/wiki/Pentagonal_tiling

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Does every computer search qualify as "experimental math"? – Gerry Myerson Aug 30 '15 at 23:24
@Gerry Myerson: Yes. – Dan Romik Aug 30 '15 at 23:41
@Gerry Myerson: furthermore, the boundary between "normal" and "experimental" math is becoming increasingly blurry, and the distinction between experimental and "rigorous" is becoming increasingly unhelpful. E.g., computer experimentation has played a key role in most of my papers. None of these are "experimental math" papers--they all have formal theorems with human-readable proofs. But the work that led me to these theorems was largely experimental. My point is that much of normal math research is "experimental." Certainly when you do a computer search, you are doing experimental math. – Dan Romik Aug 31 '15 at 0:01

Another thing you might like to check out is Herb Wilf's very nice article: Mathematics, an experimental science in the Princeton Companion to Mathematics, in which he talks about the interplay between theory and experiment, and the many forms it can take. To give one of his examples (or rather classes of examples), if you generate a sequence of integers, you can plug it into Sloane's database and you may find that it is a known sequence, but generated in a completely different way. In that case, you have an instant interesting conjecture -- that there is a connection. But he discusses several other kinds of example. Another one that deserves to be mentioned is the Bailey-Borwein-Plouffe amazing formula for pi.

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Some of the advances regarding bounded gaps between primes following Zhang's proof were using computers and computations quite substantially. This applies to progress in polymath8, to Meynard's paper, and to progress in polymath8b. See this paper for a general perspective of these advances.

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Hida and Maeda's article "Non-abelian base change for totally real fields" in the pacific journal of mathematics, volume 181(3) in 1997 explains how the experimental data lead to the "Maeda conjecture" (which you'll find mentioned in a few questions on MO).

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The Shimura-Taniyama-Weil conjecture (now a theorem) and the Birch and Swinnerton-Dyer conjectures were based on examples.

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I think that Zagier's conjectures on polylogarithms were based on considerable amount of numerical evidence (this has led to impressive work by Beilinson-Deligne, Goncharov...).

Serre's conjecture on modularity of mod $p$ $2$-dimensional Galois representations was also made precise thanks to simultaneous theoretical advances and numerical computations (this has paved the way to the proof of Fermat's last theorem...).

To come back to the prime number theorem, I think that Euler already "proved" it long before Gauss conjectured it by differentiating $\sum_{p\leq x}\frac{1}{p}\sim \log\log x$.

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I'm surprised that no one has mentioned the Mandelbrot set yet, arguably the most famous new mathematical object of the last 30 years, at least among the general public. Benoit Mandelbrot discovered it in 1979 as a result of computer experiments. He says that when he first saw it he was so surprised by its appearance that he thought it must be the result of a computer malfunction. In his book Fractals and Chaos, Mandelbrot argues that his discovery of the Mandelbrot set contributed to the revival of experimental mathematics and led to a general change in the attitude of mathematicians to experiments in mathematics. On page 25 he writes:

"The culture of mathematics during the 1960s and 1970s

Within that culture the Mandelbrot set could not have been discovered. Hence its discovery marked a historical departure. Today -- but not yesterday -- only a minority among mathematicians would agree with the opinion due to someone who did not discover that set, that the study of M reflects "a rather infantile and somewhat dull mathematical sensibility" (Brooks 1989)."

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My understanding is that John Hubbard discovered the set and described it to Mandelbrot, who had access to much better computers. – Victor Protsak Jun 6 '10 at 19:29
The Mandelbrot set was first discovered in 1978 by Robert Brooks and Peter Matelski (The dynamics of 2-generator subgroups of PSL(2,C), in "Riemann Surfaces and Related Topics", ed. Kra and Maskit, Ann. Math. Stud. 97, 65–71, ISBN 0-691-08264-2) They drew a crude but recognizable picture of it in their paper. So the statement that it could not have been discovered in the mathematical culture of the 1970s is simply false: it WAS discovered in the mathematical culture of the 1970s! – Richard Borcherds Aug 15 '10 at 14:12
Thanks for the correction. The paper by Brooks and Matelski is available online. The picture of the Mandelbrot set is on the second last page. math.harvard.edu/archive/118r_spring_05/docs/brooksmatelski.pdf – Marko Amnell Aug 15 '10 at 18:10
Another interesting point is that Mandelbrot thought originally that the Mandelbrot set was disconnected, and his computer pictures showed isolated "islands". The editor of the journal apparently thought these were specs of dust, and carefully removed them all ... in the copy of the paper that Mandelbrot sent to Hubbard, apparently Mandelbrot drew them back in by pencil. :) – Lasse Rempe-Gillen Sep 24 '15 at 11:51
It's maybe worth making it explicit that the Brooks whom Mandelbrot quotes disapprovingly is the same Brooks who actually did discover the "Mandelbrot set" before Mandelbrot did, which makes his characterization of Brooks as "someone who did not discover that set" particularly obnoxious. – Gareth McCaughan Dec 11 '15 at 12:13

Didn't the work of Candès and Tao on compressed sensing begin by a computer experiment by Candès that gave results "too good to be true"? See: http://www.ams.org/samplings/math-history/hap7-pixel.pdf

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John Conway initially thought that his cellular automata game of Life could not lead to an unlimited number of active cells starting from a finite number of cells.

Computer experimentation by Bill Gosper led to the discovery in 1970 of the Gosper Glider Gun, a finite collection of cells which leads to a recurring state which continues to emit "spaceships" or "bullets" which continue out into the periphery infinitely. I can't find a link to the specifics of the discovery beyond the remark that Gosper won a 50$US reward from Conway for making this finding. The glider gun was also used in constructions involving the manipulations of the "bullets" in such a way as to form a computational apparatus, ultimately proving the Turing-completeness of Conway's Life. I do know that a lot of random configuration discovery was done for the Life cellular automaton and for other CA by creating random patterns, then masking the lattice with a pattern which blanked out the peripheral cell values and left the random pattern alone in the center of the screen, and then allowed the CA rules to run. Escapees such as gliders would continue their motion into the blanked areas and be easily discerned in a simulation. The first glider-type pattern was discovered by tracing the evolution of the finite starting patterns. David Eppstein also appears to have written a program that helps in finding these spaceship types of pattern, which should also count as mathematical discoveries and exploration via computer programs. :gfind A program by David Eppstein which uses de Bruijn graphs to search for new spaceships. It was with gfind that Eppstein found the weekender, and Paul Tooke later used it to find the dragon. It is available at http://www.ics.uci.edu/~eppstein/ca/gfind.c (C source code only). - I'm hardly the only one to have written programs to search for CA patterns. See ics.uci.edu/~eppstein/ca/search.html for a partial listing of others. – David Eppstein Sep 2 '10 at 17:21 In some simple cases it is even possible to both guess some results and prove them too experimentally. I have done this with some Hankel determinants. Suppose you want to compute the Hankel determinant$\det \left( {a_{i + j} } \right)_{i,j = 0}^{n - 1} $of a sequence$(a_n )$. If no Hankel determinant vanishes then you can compute polynomials$p_n (x)$which are orthogonal with respect to the linear functional$F$on the polynomials defined by$F(x^n ) = a_n .$By Favard's theorem there exist numbers$s_n ,t_n $such that$p_{n + 2} (x) = (x - s_{n + 1} )p_{n + 1} (x) - t_n p_n (x).$In some cases it is easy to guess a formula for these numbers after computing some of them. Thus also the Hankel determinant can be computed. In order to show that this guess gives the correct answer we can do the following: Define$a(n,j) = a(n - 1,j - 1) + s_j a(n - 1,j) + t_j a(n - 1,j + 1)$with$a(0,j) = \delta _{j,0} .$By the underlying theory it suffices to show that$a(n,0) = a_n .$In order to do this we again compute$a(n,j)$for small values and try to guess a closed formula for them. In many cases we succeed. Then it suffices to verify that$a(n,j) = a(n - 1,j - 1) + s_j a(n - 1,j) + t_j a(n - 1,j + 1)$holds for the conjectured formula. - According to this obituary of Edward Lorenz in Plus Magazine, he discovered the seemingly chaotic behaviour of the Lorenz attractor when a small change (due to rounding) in the boundary conditions in a numerical simulation gave rise to hugely different solutions. - The results on Chebychev's bias is a great example. Chebyshev in 1853 noted that primes congruent to 3 modulo 4 seem to predominate over those congruent to 1. This is called Chebychev's bias. Littlewood proved in 1914 that this bias exists and also that it gets violated infinitely often, if you go far enough. The distance to be covered for the violation seemed to grow rapidly with each step. Rubinstein and Sarnak proved in 1994 that the violations have nonzero density. Here the density means the "logarithmic density", as defined in that paper. This was a quite interesting result guided at each step from considerations of experimental mathematics and computation. For more information have a look at the Rubinstein-Sarnak paper. - Weari-Phelan structure provides a counterexample to Kelvin's conjecture that the minimal area partition of space is realized by an appropriately curved bitruncated cubic honeycomb tesselation. It was discovered by computer simulations of foam in 1993. The problem of finding the minimizing partition is still open, but nowadays you can easily toy with it yourself using Surface Evolver. Who knows, maybe you will discover a better solution! - I think there may be an additional category, though relatively little-known outside my own field (design automation): genetic programming applications, or more generally the application of metaheuristics. Spector et al's "Genetic Programming for Finite Algebras" (2008) is a good example of the methodology, which seems to me to be much less about the automation of proof as such, and more about enhancing mathematicians' capacity for well-defined combinatorial searches. That said, you may want to include it, and the growing number of other GP papers in this domain, in your survey categories under (4) or (5). - One should surely also mention the Riemann hypothesis since Riemann computed a few zeros of the zeta function by hand and observed that they were on the critical line. I guess that this contributed to his statement, together with the functional equation, of course. (I hope that I did not overlook this response in the many previous answers, sorry otherwise). Another experimental result with a happy ending is the birth of random matrix theory by Wigner who guessed the circular law after computer experimennts if I am not mistaken. Both subjects have been recently linked experimentally. - There's also a fairly comprehensive website experimentalmath.info where, I guess, some further examples of EM in action can be found. - When I was an undergrad, Jon Borwein showed me the inverse symbolic calculator. You type in a number, and the inverse calculator tells you what it 'thinks' your number is, for example$\pi^2$. I thought it was a useful tool for experimenting. For example, if you have a series or integral which you can't evaluate exactly, you can evaluate it numerically and plug it into the inverse calculator to see if it spits out a 'meaningful' answer. You then have something that you can actually try to prove. This isn't a very deep application, but there is more information on this page for experimental mathematics. - If I am not mistaken, the connection of multiple zeta values to renormalized Feynman integrals was discovered through computations (by David Broadhurst). Here is a survey of ongoing computer research around this field (from 1996). - You asked for a classification of types of experimental mathematics. I would like to mention just one class that interests me greatly (which is the class that predominates in polymath5). There are many examples of mathematical statements that become easier to prove when you strengthen them, because you have much more of a clue of what sort of proof is appropriate. To give one example, consider the following two statements. 1. For every k there exists n such that every real sequence of length n has a monotonic subsequence of length k. 2. Every real sequence of length mn+1 has an increasing subsequence of length m+1 or a decreasing subsequence of length n+1. Even though 2 is a stronger statement, it is much easier to come up with the correct proof, because the nature of the bound gives you a big clue (particularly after you check easily that this bound is best possible). In research mathematics, we often find ourselves with problems like 1, and we want to convert them into problems like 2. And a fantastic way of doing that, which was not available to our mathematical forebears to anything like the same extent, is to program a computer to solve the problem by brute force (or better still, with the help of clever algorithms) in small, but not too small, cases. Right now in polymath5 we are trying to find a proof using semidefinite programming, which involves coming up with a quadratic form with certain properties. Both finding the form and proving that it has the required properties seem to be difficult, but the first task is much easier than it might have been, because for pretty large n one can use a computer to find the best possible quadratic form, and one can then find out a lot from that about which forms are likely to be good and which less good. - A fake projective plane is a smooth compact complex surface with the same Betti numbers as$\mathbb{P}^2(\mathbb{C})$. Mumford (MR0527834) built the first one in the late 70s using$p$-adic uniformization. Klingler (MR1990668) and, independently, Yeung (MR2128300 with erratum MR2559112), showed that they necessarily corresponded to arithmetic lattices in$\mathrm{PU}(2,1)$. (That they live in$\mathrm{PU}(2,1)$follows from Yau's Theorem MR0451180; they do the arithmeticity.) Prasad and Yeung (MR2289867) then gave the list of arithmetic constructions which could possible create such a beast. Using a computer experiment, Cartwright and Steger gave presentations for all the fundamental groups, giving a complete classification. They have recent announcement in Comptes Rendus #348, 11-13. Their presentations also allow one to show that the congruence subgroup property doesn't hold for certain commensurability classes of lattices for which it was previously unknown. In fact, these lattices are a few of the `simplest' examples for which the congruence subgroup problem remained mysterious. (Serre conjectures in his paper on$\mathrm{SL}_2$, MR0272790, that it fails for all$\mathbb{R}\$-rank one groups.)

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The proof of the four color theorem, the proof of the nonexistence of a projective plane of order 10, the proof of the Kepler conjecture, the search for Mersenne primes, the discovery of the Lorenz attractor and the Feigenbaum constant among others are examples. These are in the wikipedia article on experimental mathematics with other examples. There is also a journal of experimental mathematics. Also there are some videos here from June and July of 2008 about experimental mathematics. Many of them are about the Landen transformation.

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I don't think any precise theorems have fallen out yet, but the computations done by the 'Atlas of Lie groups' software of Adams, Vogan, du Cloux and many others has "completely changed the way [they] look at the field"

Edit: Jeff Adams agreed to give a lecture this semester where he would outline what surprising facts have been discovered, what conjectures have been made, what statements have been proved, etc. by looking at Atlas data. If this happens I'll post some kind of summary here.

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Thistlethwaite's discovery of links with trivial Jones polynomial (MR1831681) should be mentioned here. He apparently discovered them "during the course of a routine computer ennumeration." Later he and Eliahou and Kauffman (MR1928648) were able to recognize these links as part of an infinite family of links with trivial Jones polynomial. Further, it's my impression that the calculation that these Jones polynomials are trivial doesn't use computers.

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