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John Horton Conway is known for many achievements: Life, the three sporadic groups in the "Conway constellation," surreal numbers, his "Look-and-Say" sequence analysis, the Conway-Schneeberger $15$-theorem, the Free-Will theorem—the list goes on and on.

But he was so prolific that I bet he established many less-celebrated results not so widely known. Here is one: a surprising closed billiard-ball trajectory in a regular tetrahedron:


         
          Image from Izidor Hafner.


Q. What are other of Conway's lesser-known results?


Edit: Professor Conway passed away April 11, 2020 from complications of covid-19:

https://www.princeton.edu/news/2020/04/14/mathematician-john-horton-conway-magical-genius-known-inventing-game-life-dies-age

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    $\begingroup$ The latest issue of the Monthly has a paper by Conway, Paterson and Moscow. This lovely paper was written more than 40 years back, and published in a difficult-to-find festschrift for Lenstra. On this sad day, this article brings out yet again the sheer joy of Conway. $\endgroup$ – Lucia Apr 12 at 2:31
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    $\begingroup$ mobile.twitter.com/DavidSpergel/status/1249119023234789377 :( $\endgroup$ – Aaron Bergman Apr 12 at 2:39
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    $\begingroup$ @ToddTrimble News is circulating that he passed away because of coronavirus. $\endgroup$ – EFinat-S Apr 12 at 2:39
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    $\begingroup$ Coxeter and Conway constructed the beautiful "frieze patterns" in the 70s from polygonal triangulations, seemingly out of nowhere; their work was only "properly" understood many years later in the context of cluster algebras. (Probably this counts as a "well-known" result, though.) $\endgroup$ – Sam Hopkins Apr 12 at 3:20
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    $\begingroup$ @MarkSapir You know, some and perhaps many of his results may be buried in newsgroup posts, etc., or made known to others by means other than papers. $\endgroup$ – Todd Trimble Apr 12 at 4:10

34 Answers 34

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Conway discovered that the right triangle with sides $(1, 2, \sqrt{5})$ can be subdivided into five congruent triangles:

Subdivision of the (1, 2, $\sqrt{5}) right triangle into five similar triangles

Performing this subdivision repeatedly leads to the non-periodic “pinwheel tiling” of the plane by such triangles, in which the triangle appears in infinitely many different orientations:

Subdivision of the (1, 2, $\sqrt{5}) right triangle into five similar triangles

This tessellation is occasionally incorrectly credited to Radin¹, although Radin’s paper itself clearly attributes it to unpublished work of Conway.


  1. Radin, Charles. “The Pinwheel Tilings of the Plane.” Annals of Mathematics, vol. 139, no. 3, 1994, pp. 661–702.
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    $\begingroup$ @BlueRaja There are only countably infinitely many triangles in this tiling (it's fairly straightforward to construct a counting scheme), so the set of orientations must have measure zero. $\endgroup$ – probably_someone Apr 12 at 17:23
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    $\begingroup$ I would also wager that, say the cosine of the orientation angles are all algebraic. Maybe there's a nice way to describe them. $\endgroup$ – Kevin Casto Apr 12 at 19:17
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    $\begingroup$ Well, it’s easy to see from the relative orientation of the large triangle to the five constituents that all the angles belong to the abelian group generated by $\pi/2$ and $\arctan 2$. $\endgroup$ – Emil Jeřábek Apr 12 at 20:55
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    $\begingroup$ As a side note, if one replace the diagonal in the rectangle with the other diagonal, the tiling becomes periodic. $\endgroup$ – Nick S Apr 12 at 20:59
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    $\begingroup$ Just for completeness, and for those looking to read further on this tiling substitution, this is Conway and Radin's 'pinwheel tiling', for which there is a huge literature and still many open questions. $\endgroup$ – Dan Rust Apr 13 at 22:30
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Conway's office at Cambridge was notoriously messy. One day, he got tired of how hard he had to struggle to find a paper in there, and shut himself away for a few hours to come up with a solution to the problem. He proudly showed a sketch of his solution to Richard Guy, who said, "Congratulations, Conway – you've invented the filing cabinet."

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    $\begingroup$ twitter.com/robinhouston/status/1249302645556289537 $\endgroup$ – Andrés E. Caicedo Apr 12 at 13:07
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    $\begingroup$ Richard Guy had a surprisingly organized office. (At least compared to the other math professors at Calgary. And for being a centenarian.) Sadly, Dr Guy also passed away earlier this year. What an amazing person he was! I watched him give a talk at a conference in Alberta when he was 99. It was complete with a song and dance. $\endgroup$ – Mark Girard Apr 12 at 23:13
  • $\begingroup$ Wow, Dick guy really shut him down, he sounds like such a ... $\endgroup$ – hamsolo474 - Reinstate Monica Apr 27 at 7:11
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    $\begingroup$ @ham Guy & Conway were friends and collaborators, and I'm sure Conway appreciated the wit in Guy's remark. $\endgroup$ – Gerry Myerson Apr 27 at 9:37
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Although it is well known that Conway was able to quickly calculate the day of the week of any given date, it is less well known that one part of the algorithm is easy to remember and useful in practice: In any given year, the following dates all fall on the same day of the week: 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7, and the last day of February. For example, in 2020, all these dates fall on a Saturday. Conway, in his characteristically colorful way, would say that the Doomsday of 2020 is Saturday. Knowing this fact allows you to calculate fairly quickly in your head, with no special training, the day of the week for any date in 2020.

The full algorithm tells you how to calculate the Doomsday of any given year, but in everyday life, one is mostly interested in the current year, so you can just remember this year's Doomsday, and update that fact once a year.

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    $\begingroup$ there's also a mnemonic for the last four: "I work a nine to five job at the Seven-Eleven", although its memorability may be less in some regions of the world. $\endgroup$ – Will Sawin Apr 12 at 21:17
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    $\begingroup$ Are your dates in month/day or day/month format? :=) $\endgroup$ – o r Apr 13 at 11:57
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    $\begingroup$ He also had a much less-known mneumonic for converting between Hebrew and Gregorian dates. I wonder if I should post about that. $\endgroup$ – David Corwin Apr 13 at 19:05
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    $\begingroup$ I guess the reason for omitting pi day (3/14) is that it only works in day/month format? $\endgroup$ – bof Apr 13 at 22:57
  • $\begingroup$ @bof : You're free to add other days if you want (Boxing Day, July 4, etc.) but what is desired is a maximally easy-to-remember set that covers as much of the year as possible. $\endgroup$ – Timothy Chow Apr 13 at 23:49
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A convoluted set of discussions on the newsgroup geometry.puzzles in October and December 2001 seems to be due to Conway (the various threads were a mess), with the conclusion that the lines which bisect the area of a triangle do not all cross the centroid but instead form an envelope making up a deltoid whose area is $\frac{3}{4} \log_e(2) - \frac{1}{2} \approx 0.01986$ times the area of the original triangle, and affine transformations show this a constant for all triangles

As an illustration:

alt text

This is not difficult to show, so counts as minor and lesser known. I once asked here if there was any direct relationship between the deltoid and $$\sum_{n=1}^{\infty}\frac{1}{(4n-1)(4n)(4n+1)} = \frac{3}{4} \log_e(2) - \frac{1}{2}$$ apart from giving the same value

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How about the Conway-Gordon theorems? Any embedding of a six-clique in $\mathbb{R}^3$ contains a nontrivial link; any embedding of a seven-clique in $\mathbb{R}^3$ contains a nontrivial knot. My very first published paper was based on this!

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I love the works of Prof Conway and I am so sad because of this happening. This is not an answer, but a nice things about him that shows his attractive personality. This was told by Prof Peter Cameron in his blog:

This happened at a conference somewhere in North America. I was chairing the session at which he was to speak. When I got up to introduce him, his title had not yet been announced, and the stage had a blackboard on an easel. I said something like "The next speaker is John Conway, and no doubt he is going to tell us what he will talk about." John came onto the stage, went over to the easel, picked up the blackboard, and turned it over. On the other side were revealed five titles of talks. He said, "I am going to give one of these talks. I will count down to zero; you are to shout as loudly as you can the number of the talk you want to hear, and the chairman will judge which number is most popular."

So he did, and so I got to hear the talk I wanted to hear.

RIP John, the world is a poorer place without you.

A very nice memorial about Prof Conway by Princeton university:

https://www.princeton.edu/news/2020/04/14/mathematician-john-horton-conway-magical-genius-known-inventing-game-life-dies-age

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    $\begingroup$ I had a similar experience at Northwestern University in 1985-1986. He offered three possible talks and asked for a show of hands. $\endgroup$ – Philip Ehrlich Apr 12 at 21:59
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    $\begingroup$ At Mathcamp, where he was a regular visitor for several years, he had an even more extreme version: campers would suggest and vote on a series of topics, and he would only see the list of suggestions and votes at the beginning of his talk. It was, to put it mildly, extremely impressive. $\endgroup$ – Noah Schweber Apr 12 at 22:11
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Conway and Peter Doyle found a lovely proof of Morley’s trisector theorem, using only elementary geometry. Morley’s theorem says that if you take any triangle, trisect its angles, and extend the trisectors so that adjacent trisectors meet pairwise in three points, those points always form an equilateral triangle:

illustration of Morley’s theorem

Conway-Doyle’s proof begins with the equilateral triangle in the centre, and shows how to construct an arbitrary triangle around it. The details are given in Conway’s lecture The Power of Mathematics.


It’s unusual for mathematicians to publicly discuss the provenance of joint work; but Conway was an unusual mathematician, and in a talk at MOVES 2015 he explained his view of the matter:

Peter Doyle had a rather worse proof – a distinctly worse proof – and I took his worse proof and tidied it up and made this proof. So I got this, so to speak, out of Doyle’s rather worse proof. I deliberately use language reminiscent of the language used in horse racing and so on: this proof is by Conway, out of Doyle. I’ve never dared to say that in print.

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  • $\begingroup$ thanks for sharing these fantastic notes! $\endgroup$ – user347489 Apr 14 at 23:38
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The Conway base 13 function is a function $f : \mathbb{R} \rightarrow \mathbb{R}$ that takes on every real value in every interval. It is thus discontinuous at every point.

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    $\begingroup$ Oh, I love that one. It's absolutely magnificent! $\endgroup$ – Asaf Karagila Apr 13 at 18:33
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I don't know if it's lesser known, but it is certainly not on par with some of the other results on this page.

Theorem. (Doyle–Conway) Assume $\sf ZF$. If there is a bijection between $3\times A$ and $3\times B$, then there is a bijection between $A$ and $B$.

This is nontrivial. There's no reason to a priori believe that this is true without the axiom of choice. But it is. You can find the paper on arXiv.

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    $\begingroup$ The proof in the paper is Doyle and Conway’s, but the theorem is due to Lindenbaum and Tarski (unless you believe Bernstein). $\endgroup$ – Emil Jeřábek Apr 12 at 21:20
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    $\begingroup$ @Todd Tarski talks about it. Several of their results were written decades after the fact, because the war got in the way of publication. $\endgroup$ – Andrés E. Caicedo Apr 13 at 2:54
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    $\begingroup$ @Andrés: And in the way of Lindenbaum's life, which probably hindered publication even further. $\endgroup$ – Asaf Karagila Apr 13 at 6:59
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    $\begingroup$ @ToddTrimble While the original Lindenbaum’s proof claimed in the Lindenbaum and Tarski 1926 paper is only known from hearsay, Tarski undeniably published a proof of the result in 1949. $\endgroup$ – Emil Jeřábek Apr 13 at 7:49
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    $\begingroup$ The Lindenbaum–Tarski cancellation theorem was for any positive integer $n$, not just $n=3$. $\endgroup$ – bof Apr 13 at 23:00
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Conway had an analysis of the notorious Steiner-Lehmus theorem, arguing that no "equality-chasing proof" is possible. MO user Timothy Chow initiated a discussion about Conway's analysis on the FOM list some years back; see here (where Conway's argument is quoted).

For what it's worth, Wikipedia mentions a recent (2018) article that argues a direct proof of this theorem must exist (without giving the proof however!).

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    $\begingroup$ See also John Conway and Alex Ryba, The Steiner-Lehmus Angle Bisector Theorem, Math. Gazette 98 (2014), 193-203 (also reprinted in The Best Writing on Mathematics 2015). In this article, they argue that asking whether there is a "direct proof" is actually the wrong question to ask. $\endgroup$ – Timothy Chow Apr 12 at 15:08
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Apparently back in the late 90's, Conway convinced Princeton to put in a $1.2M bid at the auction for the Archimedes Palimpsest! See, here for example. He was worried that the manuscript would be hidden again, in an inaccessible vault and unavailable to researchers, as I understand.

Of course we all know the palimpsest sold for \$2M to a then-anonymous bidder, who has since allowed research and restoration of the manuscript. So both the Princeton bid and the worries of Conway may have been moot. Further this may be sideways from other answers, in that it's more an anecdote than a mathematical result, but still, a nice footnote on the interest and advocacy of the man.

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How about the shortest paper ever written with Alexander Soifer which proved that for small enough $\epsilon>0$, in order to cover an equilateral triangle of side length $n+\epsilon$, $n^2+2$ unit equilateral triangles suffice.

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  • $\begingroup$ I'd say, side length $n+\epsilon$, for some $\epsilon>0$. $\endgroup$ – Gerry Myerson Apr 13 at 1:37
  • $\begingroup$ Thanks for your comment Gerry but isn't that precisely equivalent to $>n$ but requiring more characters? Do you have a specific reason for preferring the longer version? $\endgroup$ – Ivan Meir Apr 13 at 4:16
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    $\begingroup$ The way you have it, I would take it to mean for all side lengths greater than $n$, which would be nonsense. $\endgroup$ – Gerry Myerson Apr 13 at 5:56
  • $\begingroup$ @GerryMyerson Great, thanks for the clarification - I'll update my answer. $\endgroup$ – Ivan Meir Apr 13 at 19:16
  • $\begingroup$ That paper is reproduced in mathoverflow.net/a/29459/5340 $\endgroup$ – Zsbán Ambrus Apr 14 at 8:30
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Conway Circle

Extending the sides of a triangle as shown, the six points lie on the same circle, with center at the incenter (center of the inscribed circle). If Conway was the Euler figure of modern times, this could be likened to the discovery of the Euler line, for it could have been known to the ancients.

Just one representative of his work in "classical" geometry of the triangle. He and Steve Sigur had been writing a "definitive" book on the triangle, titled The Triangle Book, but perhaps no one was holding their breath after the untimely death of Steve (a high school math teacher who would visit Princeton each summer to collaborate on the book) in 2008. I remember seeing a few sample pages on Steve's school webpage but it's gone now.

I wish I could say more. (This particular result is from a quick Google search, so may not necessarily represent his best work in this "elementary" area.)

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  • $\begingroup$ Thanks for reminding me of this nice circle! Steve Sigur's website, or parts of it, can still be found at web.archive.org/web/20081230033236/http://www.paideiaschool.org/… . $\endgroup$ – darij grinberg Apr 14 at 17:45
  • $\begingroup$ @darijgrinberg thank you for digging this up (and I see that you contributed to many MathWorld entries)! Feel free to edit this answer. $\endgroup$ – liuyao Apr 14 at 18:09
  • $\begingroup$ @darijgrinberg: Unfortunately, I never got the people at MathWorld to edit the denominator of the expression inside the radical symbol in equation (2) of their entry about the Conway Circle: mathworld.wolfram.com/ConwayCircle.html $\endgroup$ – José Hdz. Stgo. Apr 14 at 22:11
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    $\begingroup$ @JoséHdz.Stgo.: Oh yes, something is clearly wrong with that formula. I have no hot wire to MathWorld these days; my last communication with Weisstein was ca. 2004. (I never had anything like edit rights -- "contributor" means someone exchanging emails with Weisstein.) $\endgroup$ – darij grinberg Apr 16 at 11:24
  • $\begingroup$ @José and darij: the last time I talked with Eric, it seems maintenance has not been a high priority as of late. Nevertheless, I don't think it'd hurt to send another correction. $\endgroup$ – J. M. isn't a mathematician Apr 22 at 16:35
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Conway has some well-known work around the monster simple group (which he named), such as his proposal of the Monstrous moonshine conjecture with Norton, and his simplified construction of the monster, which is sketched in a chapter near the end of SPLAG. However, the following construction is not so well-known and kind of miraculous, with additional hints of more miracles in Allcock's A monstrous proposal.

Conway conjectured the $Y_{555}$-presentation of the bimonster, namely the 2-fold wreath product $\mathbb{M} \wr 2 = (\mathbb{M} \times \mathbb{M}) \rtimes (\mathbb{Z}/2\mathbb{Z})$ of the monster. This was later proved independently by S. P. Norton and A. A. Ivanov. Here, $Y_{555}$, which he later called $\mathbb{M}_{666}$, is a connected graph with a central vertex of degree 3 attached to three chains of length 5. The corresponding infinite Coxeter group, generated by the 16 reflections, surjects to the bimonster, with kernel generated by the "spider" relation: $$(ab_1c_1ab_2c_2ab_3c_3)^{10}.$$ Here, $a$ is the reflection attached to the central vertex, and $b_i, c_i$ are reflections attached to the nearby vertices in the spokes.

Conway also noted that $Y_{555}$ embeds into the 26-vertex incidence graph of $\mathbb{P}^2(\mathbb{F}_3)$, and that the corresponding Coxeter group has a homomorphism to the bimonster that extends the $Y_{555}$-map. The kernel is given by "deflating" all free 12-gons to generate copies of $S_{12}$, instead of the affine Weyl group $\mathbb{Z}^{11} \rtimes S_{12}$. Furthermore, the symmetries of the projective plane, including the duality between points and lines, extend to automorphisms of the bimonster.

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    $\begingroup$ SPLAG = Sphere Packings, Lattices and Groups $\endgroup$ – Todd Trimble Apr 13 at 1:13
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Penney's game is a non-transitive competitive two-player coin tossing game, and a method known as Conway's algorithm provides a method for calculating the probabilities of each player winning; a description is given in Plus magazine and elsewhere. But this is not something for which I can or particularly want to remember the details.

Where I do remember the details (but seems to be less widely mentioned) is the simpler question of the expected number of tosses of a fair coin until a particular pattern appears; you might naively guess it is simply the reciprocal of the probability the pattern appears immediately and for the pattern HHHHHT this is correct, being an expected $\frac1 {2^{-6}}=64$ tosses. But for the same length pattern HHHHHH it is almost twice as high at $126$.

Here Conway's algorithm for calculating the expectation is easier to remember: you see whether the length $n$ string on the left of the pattern matches the length $n$ string on the right; if so then add $2^n$ to the result (clearly this happens at least when $n$ is the full length since the string matches itself).

So for example

  • HHHHHH has $2^1+2^2+2^3+2^4+2^5+2^6=126$ expected tosses because everything matches
  • HHHHHT has $2^6=64$ expected tosses because only the full length matches
  • HHTHHH has $2^1+2^2+2^6=70$ expected tosses (the matches are H, HH and HHTHHH)
  • HHTHHT has $2^3+2^6=72$ expected tosses (the matches are HHT and HHTHHT).

For me the nice part of this is that it does not have to involve coins. Dice work too by changing $2^n$ to $6^n$. So throwing the pattern $1\, 1\, 5\, 1\, 1\, 5$ has an expected number of $6^3+6^6=46872$ throws until the pattern appears. Neat and easy.

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    $\begingroup$ Nice. I teach this in my class. And the intuitive reason that if a pattern matches one of its shifts it has a higher propensity to occur in clusters, delaying the first occurrence, in expectation. $\endgroup$ – kodlu Apr 15 at 3:40
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The FRACTRAN esoteric programming language.

Although it is related with computer languages, it is not a traditional one, because it is based more on mathematical properties than on typical programming structures

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Conway studied the following recurrence relation (OEIS), purportedly studied originally by Hofstadter (of G.E.B. fame):

$$a(k)=a(a(k-1))+a(k-(a(k-1)))$$

with initial conditions $a(1)=a(2)=1$.

HC sequence

(image from MathWorld)

Conway was able to show that

$$\lim_{k\to\infty}\frac{a(k)}{k}=\frac12$$

He offered a \$10,000 prize to anyone who could discover a value of $k$ such that

$$\left|\frac{a(j)}{j}-\frac12\right|<\frac1{20},\quad j > k$$

Collin Mallows from Bell Labs found $k=3173375556$, 34 days after Conway's initial talk on the sequence, and the prize was awarded by Conway after "adjusting" it to the "intended" value of \$1,000.

(See also this and this.)

Conway caricature

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  • $\begingroup$ There's no "k" in the inequality? $\endgroup$ – Quantum Mechanic Apr 22 at 16:25
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    $\begingroup$ @Quant, read the second condition as "for all $j$ greater than $k$". $\endgroup$ – J. M. isn't a mathematician Apr 22 at 16:32
  • $\begingroup$ OK, thanks, but isn't math a precise language? =D $\endgroup$ – Quantum Mechanic Apr 22 at 20:23
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    $\begingroup$ @Quan Indeed, so I'm not certain why you missed the $k$. $\endgroup$ – J. M. isn't a mathematician Apr 23 at 0:26
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There's one I originally learned about in this excellent answer here at Math Overflow.

Complemented modular lattices satisfying a finiteness condition are exactly the lattice of subspaces of projective spaces. This raises the question of whether we can reverse the process, and associate a geometry with every modular lattice satisfying the same finiteness condition. There are several versions of this idea, but one particularly simple one is found in Benson and Conway, Diagrams for Modular Lattices.

All of the versions share two basic ideas. We already have one clue for what a geometry should look like for a distributive lattice by considering Birkhoff's representation theorem -- join-irreducible elements are points, and these points have a natural partial order on them. What's new in the modular case is that we also have lines, which are when you have three or more join-irreducible elements such that any two of them have the same join. A complete version of this idea was already found in Faigle and Hermann, but Benson and Conway is essentially a rediscovery, but the paper itself explains the idea very clearly.

Since Conway was more famous for his work on the other kind of lattice, I was curious how many of them were about this kind of lattice. Based on a quick search of paper titles it looks like the answer is: one.

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Conway's Soldiers. And an interesting speial case Reaching row 5 in Solitaire Army.

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  • $\begingroup$ This is amazing. I didn't know about the version with infinite pegs $\endgroup$ – godelian Apr 15 at 14:53
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Conway had a more intuitive, although informal, proof of classification of compact surfaces, called the "ZIP proof", where ZIP stands for "Zero Irrelevancy Proof".

https://web.archive.org/web/20100612090500/http://new.math.uiuc.edu/zipproof/zipproof.pdf

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    $\begingroup$ Francis, George K., and Jeffrey R. Weeks. "Conway's ZIP proof." American Mathematical Monthly 106, no. 5 (1999): 393-399. $\endgroup$ – Joseph O'Rourke Apr 13 at 12:53
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The Angel problem is an interesting contribution to the pursuit-evasion branch of game theory, one of those where Conway laid out initial results, and playfully managed to spark further interest resulting in stronger bounds.

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The moving sofa problem and the Conway car

Conway worked on the moving sofa problem (find the shape of the largest sofa that can turn a right-angle corner in a corridor).

In Another Fine Math You've Got Me Into, Stewart writes:

« You’re in trouble,” said Wormstein. “You’ve landed yourself with an old chestnut and it’s a tough nut to crack. Nobody even knows where the question came from. Certainly John Horton Conway asked it in the ‘60s, but it’s probably a lot older. At that time the object being moved was a piano, but in view of the obvious piano-sofa isomorphism I think we can conclude that the optimal piano must have the same shape as the optimal sofa. The first published reference that I know is by Leo Moser in 1966. The shape you found [Figure 116] was published soon after by J. M. Hammersley, as part of a tirade against ‘Modern Mathematics,’ and he conjectured that it is optimal. But at a meeting on convexity theory in Copenhagen (some say Ann Arbor) a group of seven mathematicians, including Conway, G. C. Shephard, and possibly Moser, did some informal work on the problem. In fact they worked on seven different variations—one each!” Two are shown in Figure 117; you might like to think about them for yourselves. “And they quickly proved that Hammersley’s answer is not optimal, much as you did.”

In his proposed optimal solution (Geometriae Dedicata volume 42, pages 267–283 (1992)), Gerver cites his private correspondence with Conway.

The variation alluded to by Stewart and considered by Conway is the following: what is the optimal shape of a car that can turn around at a T-junction. The exact solution is, I think, unknown, but the solution is named the Conway car. See Stewart (loc. cit.) and Gibbs : A Computational Study of Sofas and Cars.

(I learned all this from my daughter's project on the topic.)

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Art Benjamin shared Conway's smart methods for finding by hand small prime factors of 3- and 4-digit numbers in Factoring Numbers with Conway's 150 Method, College Mathematics Journal 49 (2018) 122-125. In the acknowledgment, he thanks Conway "for being such a large prime factor in the mathematics community."

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Many years ago, Conway told me that during his high school years he kept a notebook of his discoveries in triangle geometry. Much later he introduced "Conway triangle notation"--see MathWorld for the standard version and Wikipedia for extensions.

Conway once intended to publish a triangle-shaped triangle book, as recalled by Richard Guy (https://arxiv.org/pdf/1910.03379.pdf): "This might have been titled The Triangle Book, except that John Conway already has a project in hand for such a book. Indeed, Conway’s book might well have been completed but for the tragically early death of Steve Sigur. It might also have been finished, had I been in closer proximity to John."

In addition to the Conway circle (https://mathworld.wolfram.com/ConwayCircle.html), there are also several Conway triangles and a Conway point: see X(384) in the Encyclopedia of Triangle Centers (https://faculty.evansville.edu/ck6/encyclopedia/ETC.html). The Conway point, among the named points on the Euler line of a triangle, has remarkably simple barycentric coordinates:

$$a^4+b^2c^2: b^4+c^2a^2 : c^4+a^2 b^2$$

I'll mention one more of Conway's contributions to triangle geometry: extraversion. Conway wrote, "There's a pun, of course, since I invented the term." Extraversion involves "extraverting" a triangle or turning it inside out, but it also produces "extra versions" of various entities. (from Katherine Merow's "Let's Bring Back That Gee-om-met-tree! (https://www.maa.org/let-s-bring-back-that-gee-om-met-tree).

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    $\begingroup$ Here's Conway himself talking about extraversion in 2015: youtu.be/O1GhzHmjpDQ $\endgroup$ – liuyao Apr 16 at 20:45
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The paper by John H. Conway and Joseph Shipman on "extreme" proofs of irrationality of $\sqrt{2}$,

"We shouldn’t speak of ‘‘the best’’ proof, because different people will value proofs in different ways. [...] It is enjoyable and instructive to find proofs that are optimal with respect to one or more such value functions [...] Indeed, because at any given time there are only finitely many known proofs, we may think of them as lying in a polyhedron [...] and the value functions as linear functionals, as in optimization theory, so that any value function must be maximized at some vertex. We shall call the proofs at the vertices of this polygon the extreme proofs.

Terence Tao mentions this paper here, and describes his interaction with some of Conway's contributions to mathematics and with Conway himself. He closes his post with

Conway was arguably an extreme point in the convex hull of all mathematicians. He will very much be missed.

ADDENDUM:

CONWAY published an interesting paper with R.H. Hardin, and N.J.A. Sloane regarding Packings in Grassmannian Space and it were adressed this question how should N n-dimensional subspaces of m-dimensional Euclidean space be arranged so that they are as far apart as possible? , He gives a way to describe $n$-dimensional subspaces of $m$-space as points on a sphere in dimension $(m-1)(m+2)/2$

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    $\begingroup$ That blog post by Tao just seemed like he was trying to turn the conversation back to himself, he said very little about Conway's concrete achievements. $\endgroup$ – Hollis Williams Apr 14 at 15:22
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    $\begingroup$ I am sorry but you said (I quote) 'Tao said and described here all Conway contributions to mathematics [sic]'. In that post he says almost nothing about Conway's contributions and admits lack of familiarity with most of his work, so what you said is not accurate. $\endgroup$ – Hollis Williams Apr 14 at 16:59
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    $\begingroup$ Tom, I'm sorry, but that sounds like a pretty uncharitable reading of Tao's post. I read it more as a brief personal snapshot of a genius. I think many people here may have reminiscences of the man in that spirit. $\endgroup$ – Todd Trimble Apr 15 at 0:09
  • $\begingroup$ @ToddTrimble, Really Tao is a humbel man and am sorry that Tom missed that Tao is the first man who is probably constructed MO and he has a huge contributions here and out of this space like his blog $\endgroup$ – zeraoulia rafik Apr 15 at 0:54
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    $\begingroup$ Made some substantial edits to the post to ease its reading and clarify the point of the paper being mentioned. $\endgroup$ – Andrés E. Caicedo Apr 15 at 13:39
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John Conway considered himself a classical geometer, so it seems good to mention that in 1965, he and Michael Guy discovered an anomalous uniform 4-polytope called the Grand Antiprism (pictured here with Jenn3D).

Grand Antiprism Jenn3D

It is a beautiful object with two dual rings of 10 pentagonal antiprisms, connected to each other with 300 tetrahedra. One way to construct it is by diminishing the regular 600-cell.

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John Conway and Neil Sloane collaborated often (at least 55 times by mathscinet's count). One observation they made together answered a previously unanswered question in lattice theory, namely whether there are lattices which are generated by their minimal vectors which have the additional property that the minimal vectors do not contain a basis for the lattice.

They showed that such lattices appear in dimensions as small as $d=11$ by an explicit construction. Later Jacques Martinet and Achill Schürmann discovered a new example in dimension $d=10$ and proved that phenomenon cannot happen for $d\leq 9$ settling the question of for which dimensions lattices of the above type may exist.

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In the theory of formal languages, Conway's problem asks, if the greatest solution $X$ of $LX = XL$, for some finite language $L$, is regular. Now, we know that this does not have to be the case, but it was an open problem for many years.

It goes back to his book Regular algebra and finite machines, which grew out of the work of one of his PhD students. In the book, he gave a proof of Parikh's theorem that is quite short and elegant. His student published the proof. The original proof is very long and technical.

I studied mathematics, and did some group theory classes. So surely I knew about John Conway. As I started my PhD in theoretical computer science, it was a little bit surprising to find out that he had done some work in formal language theory. The book has a somewhat unconventional take on it. As far as I remember, in it he introduced biregular relations, which seemed to be quite similar to what was later introduced as an algebraic treatment of transductions. Also, he introduced the factor matrix of some regular language, which is also called the universal automaton.

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This is a puzzle rather than a theorem, but I think it fits in this wonderful list:

Conway’s Wizards, as discussed here by Tanya Khovanova.

Last night I sat behind two wizards on a bus, and overheard the following:

A: “I have a positive integral number of children, whose ages are positive integers, the sum of which is the number of this bus, while the product is my own age.”

B: “How interesting! Perhaps if you told me your age and the number of your children, I could work out their individual ages?”

A: “No.”

B: “Aha! AT LAST I know how old you are!”

Now what was the number of the bus?

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As a graduate student, Conway proved that any integer is the sum of at most $37$ integer $5$-th powers.

I think I read this in Siobhan Roberts' Genius at Play, which I cannot access now. Otherwise, I have not been able to find a citation for this result. I would appreciate any confirmation of this $5$-th powers theorem.

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