## The problem with combinatorics textbooks

Every now and then I think about writing a graduate textbook in Combinatorics, based on some topics courses I have taught. I scan my extensive lecture notes, think about how much time it would take, and whether there is even a demand for this kind of effort. Five minutes later I would always remember that YOLO, deeply exhale and won’t think about it for a while.

**What’s wrong with Combinatorics?**

To illustrate the difficulty, let me begin with two quotes which contradict each other in the most illuminating way. First, from the Foreword by **Richard Stanley** on (his former student) Miklós Bóna’s “*A Walk Through Combinatorics*” textbook:

The subject of combinatorics is so vast that the author of a textbook faces a difficult decision as to what topics to include. There is no more-or-less canonical corpus as in such other subjects as number theory and complex variable theory. [here]

Second, from the Preface by **Kyle Petersen** (and Stanley’s academic descendant) in his elegant “*Inquiry-Based Enumerative* *Combinatorics*” textbook:

Combinatorics is a very broad subject, so the difficulty in writing about the subject is not what to include, but rather what to exclude. Which hundred problems should we choose? [here]

Now that this is all clear, you can probably insert your own joke about importance of teaching inclusion-exclusion. But I think the issue is a bit deeper than that.

I’ve been thinking about this when updating my “*What is Combinatorics*” quotation page (see also my old blog post on this). You can see a complete divergence of points of view on how to answer this question. Some make the definition or description to be very broad (sometimes even ridiculously broad), some relatively narrow, some are overly positive, while others are revoltingly negative. And some basically give up and say, in effect “it is what it is”. This may seem puzzling, but if you concentrate on the narrow definitions and ignore the rest, a picture emerges.

Clearly, these people are not talking about the same area. They are talking about sub-areas of Combinatorics that they know well, that they happen to learn or work on, and that they happen to like or dislike. Somebody made a choice what part of Combinatorics to teach them. They made a choice what further parts of Combinatorics to learn. These choices are increasingly country or culture dependent, and became formative in people’s mind. And they project their views of these parts of Combinatorics on the whole field.

So my point is — there is no right answer to “*What is Combinatorics?*“, in a sense that all these opinions are biased to some degree by personal education and experience. Combinatorics is just too broad of a category to describe. It’s a bit like asking “*what is good food?*” — the answers would be either broad and bland, or interesting but very culture-specific.

**Courses and textbooks**

How should one resolve the issue raised above? I think the answer is simple. Stop claiming that *Combinatorics*, or worse, *Discrete Mathematics*, is one subject. That’s not true and hasn’t been true for a while. I talked about this in my “Unity of Combinatorics” book review. Combinatorics is comprised of many sub-areas, see the *Wikipedia article* I discussed here (long ago). Just accept it.

As a consequence, you should never teach a “Combinatorics” course. **Never!** Especially to graduate students, but to undergraduates as well. Teach courses in any and all of these subjects: *Enumerative Combinatorics*, *Graph Theory*, *Probabilistic Combinatorics*, *Discrete Geometry*, *Algebraic Combinatorics*, *Arithmetic Combinatorics*, etc. Whether introductory or advanced versions of these courses, there is plenty of material for each such course.

Stop using these broad “a little bit about everything” combinatorics textbooks which also tend to be bulky, expensive and shallow. It just doesn’t make sense to teach both the *five color theorem* and the *Catalan numbers* (see also here) in the same course. In fact, this is a disservice to both the students and the area. Different students want to know about different aspects of Combinatorics. Thus, if you are teaching the same numbered undergraduate course every semester you can just split it into two or three, and fix different syllabi in advance. The students will sort themselves out and chose courses they are most interested in.

**My own teaching**

At UCLA, with the help of the Department, we split one Combinatorics course into two titled “Graph Theory” and “Enumerative Combinatorics”. They are broader, in fact, than the titles suggest — see Math 180 and Math 184 here. The former turned out to be quite a bit more popular among many applied math and non-math majors, especially those interested in CS, engineering, data science, etc., but also from social sciences. Math majors tend to know a lot of this material and flock to the latter course. I am not saying you should do the same — this is just an example of what *can *be done.

I remember going through a long list of undergraduate combinatorics textbooks a few years ago, and found surprisingly little choice for the enumerative/algebraic courses. Of the ones I liked, let me single out Bóna’s “*Introduction to Enumerative and Analytic Combinatorics“* and Stanley’s “*Algebraic Combinatorics*“. We now use both at UCLA. There are also many good *Graph Theory* course textbooks of all levels, of course.

Similarly, for graduate courses, make sure you make the subject relatively narrow and clearly defined. Like a topics class, except accessible to beginning graduate students. Low entry barrier is an advantage Combinatorics has over other areas, so use it. To give examples from my own teaching, see unedited notes from my graduate courses:

*Combinatorics of posets* (Fall 2020)

*Combinatorics and Probability on groups* (Spring 2020)

*Algebraic Combinatorics* (Winter 2019)

*Discrete and Polyhedral Geometry* (Fall 2018) This is based on my book. See also videos of selected topics (in Russian).

Combinatorics of Integer Sequences (Fall 2016)

*Combinatorics of Words *(Fall 2014)

*Tilings* (Winter 2013, lecture-by-lecture refs only)

#### In summary

In my experience, the more specific you make the combinatorics course the more interesting it is to the students. Don’t be afraid that the course would appear be too narrow or too advanced. That’s a stigma from the past. You create a good course and the students will quickly figure it out. They do have their own FB and other chat groups, and spread the news much faster than you could imagine…

Unfortunately, there is often no good textbook to cover what you want. So you might have to work a little harder harder to scout the material from papers, monographs, etc. In the internet era this is easier than ever. In fact, many extensive lecture notes are already available on the web. Eventually, all the appropriate textbooks will be written. As I mentioned before, this is one of the very few silver linings of the pandemic…

**P.S. ** (July 8, 2021) I should have mentioned that in addition to “a little bit about everything” textbooks, there are also “a lot about everything” doorstopper size volumes. I sort of don’t think of them as textbooks at all, more like mixtures of a reference guide, encyclopedia and teacher’s manual. Since even the thought of teaching from such books overwhelms the senses, I don’t expect them to be widely adopted.

Having said that, these voluminous textbooks can be incredibly valuable to both the students and the instructor as a source of interesting supplementary material. Let me single out an excellent recent “*Combinatorial Mathematics*” by Doug West written in the same clear and concise style as his earlier “*Introduction to Graph Theory*“. Priced modestly (for 991 pages), I recommend it as “further reading” for all combinatorics courses, even though I strongly disagree with the second sentence of the Preface, per my earlier blog post.

## Why you shouldn’t be too pessimistic

In our math research we make countless choices. We chose a problem to work on, decide whether its claim is true or false, what tools to use, what earlier papers to study which might prove useful, who to collaborate with, which computer experiments might be helpful, etc. Choices, choices, choices… Most our choices are private. Others are public. This blog is about wrong public choices that I made misjudging some conjectures by being overly pessimistic.

#### The meaning of conjectures

As I have written before, conjectures are crucial to the developments of mathematics and to my own work in particular. The concept itself is difficult, however. While traditionally conjectures are viewed as some sort of “*unproven laws of nature*“, that comparison is widely misleading as many conjectures are descriptive rather than quantitative. To understand this, note the stark contrast with experimental physics, as many mathematical conjectures are not particularly testable yet remain quite interesting. For example, if someone conjectures there are infinitely many *Fermat primes*, the only way to dissuade such person is to actually disprove the claim.

There is also an important social aspect of conjecture making. For a person who poses a conjecture, there is a certain clairvoyance respected by other people in the area. Predictions are never easy, especially of a precise technical nature, so some bravery or self-assuredness is required. Note that social capital is spent every time a conjecture is posed. In fact, a lot of it is lost when it’s refuted, you come out even if it’s proved relatively quickly, and you gain only if the conjecture becomes popular or proved possibly many years later. There is also a “*boy who cried wolf*” aspect for people who make too many conjectures of dubious quality — people will just tune out.

Now, for the person working on a conjecture, there is also a *betting aspect* one cannot ignore. As in, are you sure you are working in the right direction? Perhaps, the conjecture is simply *false *and you are wasting your time… I wrote about this all before in the post linked above, and the life/career implications on the solver are obvious. The success in solving a well known conjecture is often regarded much higher than a comparable result nobody asked about. This may seem unfair, and there is a bit of celebrity culture here. Thinks about it this way — two lead actors can have similar acting skills, but the one who is a star will usually attract a much larger audience…

#### Stories of conjectures

Not unlike what happens to papers and mathematical results, conjectures also have stories worth telling, even if these stories are rarely discussed at length. In fact, these “** conjecture stories**” fall into a few types. This is a little bit similar to the “

*types of scientific papers*” meme, but more detailed. Let me list a few scenarios, from the least to the most mathematically helpful:

**(1)** * Wishful thinking*. Say, you are working on a major open problem. You realize that a famous conjecture

**A**follows from a combination of three conjectures

**B**,

**C**and

**D**whose sole motivation is their applications to

**A**. Some of these smaller conjectures are beyond the existing technology in the area and cannot be checked computationally beyond a few special cases. You then declare that this to be your “

*program*” and prove a small special case of

**C**. Somebody points out that

**D**is trivially false. You shrug, replace it with a weaker

**D’**which suffices for your program but is harder to disprove. Somebody writes a long state of the art paper disproving

**D’**. You shrug again and suggest an even weaker conjecture

**D”**. Everyone else shrugs and moves on.

**(2)** ** Reconfirming long held beliefs**. You are working in a major field of study aiming to prove a famous open problem

**A**. Over the years you proved a number of special cases of

**A**and became one the leaders of the area. You are very optimistic about

**A**discussing it in numerous talks and papers. Suddenly

**A**is disproved in some esoteric situations, undermining the motivation of much of your older and ongoing work. So you propose a weaker conjecture

**A’**as a replacement for

**A**in an effort to salvage both the field and your reputation. This makes happy everyone in the area and they completely ignore the disproof of

**A**from this point on, pretending it’s completely irrelevant. Meanwhile, they replace

**A**with

**A’**in all subsequent papers and beamer talk slides.

**(3)** ** Accidental discovery.** In your ongoing work you stumble at a coincidence. It seem, all objects of a certain kind have some additional property making them “

*nice*“. You are clueless why would that be true, since being

*nice*belongs to another area

**X**. Being

*nice*is also too abstract to be checked easily on a computer. You consult a colleague working in

**X**whether this is obvious/plausible/can be proved and receive No/Yes/Maybe answers to these three questions. You are either unable to prove the property or uninterested in problem, or don’t know much about

**X**. So you mention it in the

*Final Remarks*section of your latest paper in vain hope somebody reads it. For a few years, every time you meet somebody working in

**X**you mention to them your “nice conjecture”, so much that people laugh at you behind your back.

** (4) Strong computational evidence.** You are doing computer experiments related to your work. Suddenly certain numbers appear to have an unexpectedly nice formula or a generating function. You check with OEIS and the sequence is there indeed, but not with the meaning you wanted. You use the “

*scientific method*” to get a few more terms and they indeed support your conjectural formula. Convinced this is not an instance of the “

*strong law of small numbers*“, you state the formula as a conjecture.

**(5) Being contrarian. ** You think deeply about famous conjecture

**A**. Not only your realize that there is no way one can approach

**A**in full generality, but also that it contradicts some intuition you have about the area. However,

**A**was stated by a very influential person

*N*and many people believe in

**A**proving it in a number of small special cases. You want to state a

**non-A**conjecture, but realize the inevitable PR disaster of people directly comparing you to

*N*. So you either state that you don’t believe in

**A**, or that you believe in a conjecture

**B**which is either slightly stronger or slightly weaker than

**non-A**, hoping the history will prove you right.

**(6) Being inspirational.** You think deeply about the area and realize that there is a fundamental principle underlying certain structures in your work. Formalizing this principle requires a great deal of effort and results in a conjecture

**A**. The conjecture leads to a large body of work by many people, even some counterexamples in esoteric situations, leading to various fixes such as

**A’**. But at that point

**A’**is no longer the goal but more of a direction in which people work proving a number of

**A**-related results.

Obviously, there are many other possible stories, while some stories are are a mixture of several of these.

#### Why do I care? Why now?

In the past few years I’ve been collecting references to my papers which solve or make some progress towards my conjectures and open problems, putting links to them on my research page. Turns out, over the years I made a lot of those. Even more surprisingly, there are quite a few papers which address them. Here is a small sampler, in random order:

**(1)** Scott Sheffield proved my *ribbon tilings *conjecture.

**(2)** Alex Lubotzky proved my conjecture on *random generation* of a finite group.

**(3)** Our generalized *loop-erased random walk* conjecture (joint with Igor Gorodezky) was recently proved by Heng Guo and Mark Jerrum.

**(4)** Our *Young tableau bijections* conjecture (joint with Ernesto Vallejo) was resolved by André Henriques and Joel Kamnitzer.

**(5)** My *size Ramsey numbers* conjecture led to a series of papers, and was completely resolved only recently by Nemanja Draganić, Michael Krivelevich and Rajko Nenadov.

**(6)** One of my *partition bijection* problems was resolved by Byungchan Kim.

The reason I started collecting these links is kind of interesting. I was very impressed with George Lusztig and Richard Stanley‘s lengthy writeups about their collected papers that I mentioned in this blog post. While I don’t mean to compare myself to these giants, I figured the casual reader might want to know if a conjecture in some paper had been resolved. Thus the links on my website. I recommend others also do this, as a navigational tool.

#### What gives?

Well, looks like none of my conjectures have been disproved yet. That’s a good news, I suppose. However, by going over my past research work I did discover that on three occasions when I was thinking about other people’s conjectures, I was much too negative. This is probably the result of my general inclination towards “*negative thinking*“, but each story is worth telling.

**( i)** Many years ago, I spent some time thinking about

*Babai’s conjecture*which states that there are universal constants

*C*,

*c*>0, such that for every simple group

*G*and a generating set

*S*, the diameter of the

*Cayley graph*Cay(

*G,S*) is at most

*C*(log |

*G*|)

^{c}. There has been a great deal of work on this problem, see e.g. this paper by Sean Eberhard and Urban Jezernik which has an overview and references.

Now, I was thinking about the case of the symmetric group trying to apply *arithmetic combinatorics* ideas and going nowhere. In my frustration, in a talk I gave (Galway, 2009), I wrote on the slides that “there is much less hope” to resolve Babai’s conjecture for *A _{n} *than for simple groups of Lie type or bounded rank. Now, strictly speaking that judgement was correct, but much too gloomy. Soon after, Ákos Seress and Harald Helfgott

**a remarkable quasi-polynomial upper bound in this case. To my embarrassment, they referenced my slides as a validation of the importance of their work.**

*proved*Of course, Babai’s conjecture is very far from being resolved for *A _{n}*. In fact, it is possible that the diameter is always

*O*(

*n*

^{2}). We just have no idea. For simple groups of Lie type or large rank the existing worst case diameter bounds are exponential and much too weak compared to the desired bound. As Eberhard and Jezernik amusingly wrote in the paper linked above, “

*we are still exponentially stupid*“…

**( ii)** When he was my postdoc at UCLA, Alejandro Morales told me about a curious conjecture in this paper (Conjecture 5.1), which claimed that the number of certain nonsingular matrices over the finite field

**F**

*is polynomial in*

_{q}*q*with positive coefficients. He and coauthors proved the conjecture is some special cases, but it was wide open in full generality.

Now, I thought about this type of problems before and was very skeptical. I spent a few days working on the problem to see if any of my tools can disprove it, and failed miserably. But in my stubbornness I remained negative and suggested to Alejandro that he should drop the problem, or at least stop trying to prove rather than disprove the conjecture. I was wrong to do that.

Luckily, Alejandro ignored my suggestion and soon after ** proved **the polynomial part of the conjecture together with Joel Lewis. Their proof is quite elegant and uses certain recurrences coming from the

*rook theory*. These recurrences also allow a fast computation of these polynomials. Consequently, the authors made a number of computer experiments and

*the positivity of coefficients part of the conjecture. So the moral is not to be so negative. Sometimes you need to prove a positive result first before moving to the dark side.*

**disproved****( iii)** The final story is about the beautiful

*Benjamini conjecture*in probabilistic combinatorics. Roughly speaking, it says that for every finite vertex transitive graph

*G*on

*n*vertices and diameter

*O*(

*n*/log

*n*) the critical percolation constant

*p*

_{c}<1. More precisely, the conjecture claims that there is

*p*<1-ε, such that a

*p*-percolation on

*G*has a connected component of size >

*n*/2 with probability at least δ, where constants ε, δ>0 depend on the constant implied by the

*O*(*) notation, but not on

*n*. Here by “

*p*-percolation” we mean a random subgraph of

*G*with probability

*p*of keeping and 1-

*p*of deleting an edge, independently for all edges of

*G*.

Now, Itai Benjamini is a fantastic conjecture maker of the best kind, whose conjectures are both insightful and well motivated. Despite the somewhat technical claim, this conjecture is quite remarkable as it suggested a finite version of the “*p*_{c}<1″ phenomenon for infinite groups of superlinear growth. The latter is the famous *Benjamini–Schramm conjecture* (1996), which was recently ** proved **in a remarkable breakthrough by Hugo Duminil-Copin, Subhajit Goswami, Aran Raoufi, Franco Severo and Ariel Yadin. While I always believed in that conjecture and even proved a tiny special case of it, finite versions tend to be much harder in my experience.

In any event, I thought a bit about the Benjamini conjecture and talked to Itai about it. He convinced me to work on it. Together with Chis Malon, we wrote a paper proving the claim for some Cayley graphs of abelian and some more general classes of groups. Despite our best efforts, we could not prove the conjecture even for Cayley graphs of abelian groups in full generality. Benjamini noted that the conjecture is tight for products of two cyclic groups, but that justification did not sit well with me. There seemed to be no obvious way to prove the conjecture even for the Cayley graph of *S _{n}* generated by a transposition and a long cycle, despite the very small

*O*(

*n*

^{2}) diameter. So we wrote in the introduction: “In this paper we present a number of positive results toward this unexpected, and, perhaps, overly optimistic conjecture.”

As it turns out, it was us who were being overly pessimistic, even if we never actually stated that we believe the conjecture is false. Most recently, in an amazing development, Tom Hutchcroft and Matthew Tointon **proved **a slightly weaker version of the conjecture by adapting the methods of Duminil-Copin et al. They assume the *O*(*n*/(log* n*)^{c}) upper bound on the diameter which they prove is sufficient, for some universal constant *c*>1. They also extend our approach with Malon to prove the conjecture for all Cayley graphs of abelian groups. So while the Benjamini conjecture is not completely resolved, my objections to it are no longer valid.

#### Final words on this

All in all, it looks like I was never formally wrong even if I was a little dour occasionally (*Yay*!?). Turns out, some conjectures are actually true or at least likely to hold. While I continue to maintain that not enough effort is spent on trying to disprove the conjectures, it is very exciting when they are proved. * Congratulations* to Harald, Alejandro, Joel, Tom and Matthew, and posthumous congratulations to Ákos for their terrific achievements!

## The Unity of Combinatorics

I just finished my very first * book review* for the

*Notices of the AMS*. The authors are Ezra Brown and Richard Guy, and the book title is the same as the blog post. I had mixed feelings when I accepted the assignment to write this. I knew this would take a lot of work (I was wrong — it took a

*huge*amount of work). But the reason I accepted is because I strongly suspected that there is

*“unity of combinatorics”, so I wanted to be proved wrong. Here is how the book begins:*

**no**One reason why Combinatorics has been slow to become accepted as part of mainstream Mathematics is the common belief that it consists of a bag of isolated tricks, a number of areas: [very long list – IP] with little or no connection between them. We shall see that they have numerous threads weaving them together into a beautifully patterned tapestry.

Having read the book, I continue to maintain that there is no unity. The book review became a balancing act — how do you write a somewhat positive review if you don’t believe into the mission of the book? Here is the first paragraph of the portion of the review where I touch upon themes very familiar to readers of this blog:

As I see it, the whole idea of combinatorics as a “

slow to become accepted” field feels like a throwback to the long forgotten era. This attitude was unfair but reasonably common back in 1970, outright insulting and relatively uncommon in 1995, and was utterly preposterous in 2020.

After a lengthy explanation I conclude:

To finish this line of thought, it gives me no pleasure to conclude that the case for the unity of combinatorics is too weak to be taken seriously. Perhaps, the unity of mathematics as a whole is an easier claim to establish, as evident from [Stanley’s] quotes. On the other hand, this lack of unity is not necessarily a bad thing, as we would be amiss without the rich diversity of cultures, languages, open problems, tools and applications of different areas.

Enjoy the full review! And please comment on the post with your own views on this alleged “unity”.

P.S. A large part of the book is freely downloadable. I made this website for the curious reader.

**Remark** (ADDED April 17, 2021)

Ezra “Bud” Brown gave a talk on the book illustrating many of the connections I discuss in the review. This was at a memorial conference celebrating Richard Guy’s legacy. I was not aware of the video until now. Watch the whole talk.

## 2021 Abel Prize

I am overjoyed with the news of the *Abel prize* awarded to **László Lovász** and** Avi Wigderson**. You can now see three (!) Abel laureates discussing Combinatorics — follow the links in this blog post from 2019. See also Gil Kalai’s blog post for further links to lectures.

## My interview

Readers of this blog will remember my strong advocacy for taking interviews. In a surprising turn of events, Toufik Mansour interviewed me for the journal *Enumerative Combinatorics and Applications* (ECA). **Here is that interview**. Not sure if I am the right person to be interviewed, but if you want to see other Toufik’s interviews — **click here** (I mentioned some of them earlier). I am looking forward to read interviews of many more people in ECA and other journals.

**P.S.** The interview asks also about this blog, so it seems fitting to mention it here.

**Corrections:** (March 11, 2021) **1.** I misread “What three results do you consider the most influential in combinatorics during the last thirty years?” question as asking about *my own* three results that are specifically in combinatorics. Ugh, to the original question – none of *my* results would go on that list. **2.** In the pattern avoidance question, I misstated the last condition: I am asking for *e*^{c(Π)} to be non-algebraic. Sorry everyone for all the confusion!

## What if they are all wrong?

* Conjectures *are a staple of mathematics. They are everywhere, permeating every area, subarea and subsubarea. They are diverse enough to avoid a single general adjective. They come in al shapes and sizes. Some of them are famous, classical, general, important, inspirational, far-reaching, audacious, exiting or popular, while others are speculative, narrow, technical, imprecise, far-fetched, misleading or recreational. That’s a lot of beliefs about unproven claims, yet we persist in dispensing them, inadvertently revealing our experience, intuition and biases.

The conjectures also vary in attitude. Like a finish line ribbon they all appear equally vulnerable to an outsider, but in fact differ widely from race to race. *Some *are eminently reachable, the only question being who will get there first (think 100 meter dash). *Others *are barely on the horizon, requiring both great effort, variety of tools, and an extended time commitment (think ironman triathlon). The most celebrated *third type* are like those Sci-Fi space expeditions in requiring hundreds of years multigenerational commitments, often losing contact with civilization it left behind. And we can’t forget the romantic *fourth type* — like the North Star, no one actually wants to reach them, as they are largely used for navigation, to find a direction in unchartered waters.

Now, conjectures famously provide a foundation of the *scientific method*, but that’s not at all how we actually think of them in mathematics. I argued back in this pointed blog post that *citations* are the most crucial for the day to day math development, so one should take utmost care in making references. While this claim is largely uncontroversial and serves as a raison d’être for most *GoogleScholar* profiles, conjectures provide a convenient idealistic way out. Thus, it’s much more noble and virtuous to say “*I dedicated my life to the study of the XYZ Conjecture*” (even if they never publish anything), than “*I am working hard writing so many papers to gain respect of my peers, get a promotion, and provide for my family*“. Right. Obviously…

But given this apparent (true or perceived) importance of conjectures, are you sure you are using them right? * What if some/many of these conjectures are actually wrong, what then?* Should you be flying that starship if

*there is no there there*? An idealist would argue something like “

*it’s a journey, not a destination*“, but I strongly disagree. Getting closer to the truth is actually kind of important, both as a public policy and on an individual level. It is thus pretty important to get it right where we are going.

#### What *are *conjectures in mathematics?

That’s a stupid question, right? Conjectures are mathematical claims whose validity we are trying to ascertain. Is that all? Well, yes, if you don’t care if anyone will actually work on the conjecture. In other words, *something *about the conjecture needs to *interesting *and *inspiring*.

#### What makes a conjecture interesting?

This is a hard question to answer because it is as much psychological as it is mathematical. A typical answer would be “oh, because it’s old/famous/beautiful/etc.” Uhm, ok, but let’s try to be a little more formal.

One typically argues “oh, that’s because this conjecture would imply [a list of interesting claims and known results]”. Well, ok, but this is *self-referential*. We already know all those “known results”, so no need to prove them again. And these “claims” are simply other conjectures, so this is really an argument of the type “this conjecture would imply that conjecture”, so not universally convincing. One can argue: “look, this conjecture has so many interesting consequences”. But this is both subjective and unintuitive. Shouldn’t having so many interesting conjectural consequences suggest that perhaps the conjecture is too strong and likely false? And if the conjecture is likely to be false, shouldn’t this make it *uninteresting*?

Also, wouldn’t it be *interesting *if you disprove a conjecture everyone believes to be true? In some sense, wouldn’t it be even more interesting if until now everyone one was simply wrong?

None of this are new ideas, of course. For example, faced with the need to justify the “great” *BC conjecture*, or rather 123 pages of survey on the subject (which is quite interesting and doesn’t really need to be justified), the authors suddenly turned reflective. Mindful of self-referential approach which they quickly discard, they chose a different tactic:

We believe that the interest of a conjecture lies in the feeling of unity of mathematics that it entails. [M.P. Gomez Aparicio, P. Julg and A. Valette, “

The Baum-Connes conjecture“, 2019]

Huh? Shouldn’t math be about absolute truths, not feelings? Also, in my previous blog post, I mentioned Noga Alon‘s quote that Mathematics* *is already “*one unit*“. If it is, why does it need a new “*feeling of* *unity*“? Or is that like one of those new age ideas which stop being true if you don’t reinforce them at every occasion?

If you are confused at this point, welcome to the club! There is no objective way to argue what makes certain conjectures interesting. It’s all in our imagination. Nikolay Konstantinov once told me that “*mathematics is a boring subject because every statement is equivalent to saying that some set is empty*.” He meant to be provocative rather than uninspiring. But the problem he is underlying is quite serious.

#### What makes us believe a conjecture is true?

We already established that in order to argue that a conjecture is interesting we need to argue it’s also true, or at least we want to believe it to be true to have all those consequences. Note, however, that we argue that a conjecture is *true *in exactly the same way we argue it’s *interesting*: by showing that it holds is some special cases, and that it would imply other conjectures which are believed to be true because they are also checked in various special cases. So in essence, this gives “true = interesting” in most cases. Right?

This is where it gets complicated. Say, you are working on the “*abc conjecture*” which may or may not be open. You claim that it has many consequences, which makes it both likely true and interesting. One of them is the negative solution to the *Erdős–Ulam problem* about existence of a dense set in the plane with rational pairwise distances. But a positive solution to the E-U problem implies the *Harborth’s conjecture* (aka the “*integral Fáry problem*“) that every graph can be drawn in the plane with rational edge lengths. So, counterintuitively, if you follow the logic above shouldn’t you be working on a *positive solution* to Erdős–Ulam since it would both imply one conjecture and give a counterexample to another? For the record, I wouldn’t do that, just making a polemical point.

I am really hoping you see where I am going. Since there is no objective way to tell if a conjecture is true or not, and what exactly is so interesting about it, shouldn’t we discard our biases and also work towards disproving the conjecture just as hard as trying to prove it?

#### What do people say?

It’s worth starting with a general (if slightly poetic) modern description:

In mathematics, [..] great conjectures [are] sharply formulated statements that are most likely true but for which no conclusive proof has yet been found. These conjectures have deep roots and wide ramifications. The search for their solution guides a large part of mathematics. Eternal fame awaits those who conquer them first. Remarkably, mathematics has elevated the formulation of a conjecture into high art. [..] A well-chosen but unproven statement can make its author world-famous, sometimes even more so than the person providing the ultimate proof. [Robbert Dijkgraaf,

The Subtle Art of the Mathematical Conjecture, 2019]

Karl Popper thought that conjectures are foundational to science, even if somewhat idealized the efforts to disprove them:

[Great scientists] are men of bold ideas, but highly critical of their own ideas: they try to find whether their ideas are right by trying first to find whether they are not perhaps wrong. They work with bold conjectures and severe attempts at refuting their own conjectures. [Karl Popper,

Heroic Science, 1974]

Here is how he reconciled somewhat the apparent contradiction:

On the pre-scientific level we hate the very idea that we may be mistaken. So we cling dogmatically to our conjectures, as long as possible. On the scientific level, we systematically search for our mistakes. [Karl Popper, quoted by Bryan Magee, 1971]

Paul Erdős was, of course, a champion of conjectures and open problems. He joked that the purpose of life is “*proof and conjecture*” and this theme is repeatedly echoed when people write about him. It is hard to overestimate his output, which included hundreds of talks titled “*My favorite problems*“. He wrote over 180 papers with collections of conjectures and open problems (nicely assembled by *Zbl. Math*.)

Peter Sarnak has a somewhat opposite point of view, as he believes one should be extremely cautious about stating a conjecture so people don’t waste time working on it. He said once, only half-jokingly:

Since we reward people for making a right conjecture, maybe we should punish those who make a wrong conjecture. Say,

cut off their fingers. [Peter Sarnak, UCLA, c. 2012]

This is not an exact quote — I am paraphrasing from memory. Needless to say, I disagree. I don’t know how many fingers he wished Erdős should lose, since some of his conjectures were definitely disproved: one, two, three, four, five, and six. This is not me gloating, the opposite in fact. When you are stating hundreds of conjectures in the span of almost 50 years, having only a handful to be disproved is an amazing batting average. It would, however, make me happy if *Sarnak’s conjecture* is disproved someday.

Finally, there is a bit of a controversy whether conjectures are worth as much as theorems. This is aptly summarized in this quote about yet another champion of conjectures:

Louis J. Mordell [in his book review] questioned Hardy‘s assessment that Ramanujan was a man whose native talent was equal to that of Euler or Jacobi. Mordell [..] claims that one should judge a mathematician by what he has actually done, by which Mordell seems to mean, the theorems he has proved. Mordell’s assessment seems quite wrong to me. I think that a felicitous but unproved conjecture may be of much more consequence for mathematics than the proof of many a respectable theorem. [Atle Selberg, “

Reflections Around the Ramanujan Centenary“, 1988]

#### So, what’s the problem?

Well, the way I see it, the efforts made towards proving vs. disproving conjectures is greatly out of balance. Despite all the high-minded Popper’s claims about “*severe attempts at refuting their own conjectures*“, I don’t think there is much truth to that in modern math sciences. This does not mean that disproofs of famous conjectures aren’t celebrated. Sometimes they are, see below. But it’s clear to me that the proofs are celebrated more frequently, and to a much greater degree. I have only anecdotal evidence to support my claim, but bear with me.

Take prizes. Famously, Clay Math Institute gives **$1 million** for a solution of any of these major open problems. But look closely at the rules. According to the item 5b, except for the * P vs. NP problem* and the

*, it gives*

**Navier–Stokes Equation problem****(**

*nothing***$0**) for a disproof of these problems. Why, oh why?? Let’s look into CMI’s “

*primary objectives and purposes*“:

To recognize extraordinary achievements and advances in mathematical research.

So it sounds like CMI does not think that disproving the * Riemann Hypothesis* needs to be rewarded because this wouldn’t “advance mathematical research”. Surely, you are joking? Whatever happened to “

*the opposite of a profound truth may well be another profound truth*“? Why does the CMI wants to put its thumb on the scale and support only one side? Do they not want to find out the solution whatever it is? Shouldn’t they be eager to dispense with the “wrong conjecture” so as to save numerous researches from “

*advances to nowhere*“?

I am sure you can see that my blood is boiling, but let’s proceed to the * P vs. NP problem*. What if it’s

*independent of ZFC*? Clearly, CMI wouldn’t pay for proving that. Why not? It’s not like this kind of thing never happened before (see obligatory link to CH). Some people believe that (or at least they did in 2012), and some people like Scott Aaronson take this seriously enough. Wouldn’t this be a great result worthy of an award as much as the proof that

**P=NP**, or at least a

*nonconstructive proof*that

**P=NP**?

If your head is not spinning hard enough, here is another amusing quote:

Of course, it’s possible that

P vs. NPis unprovable, but that that fact itself will forever elude proof: indeed, maybe the question of the independence ofP vs. NPis itself independent of set theory, and so on ad infinitum! But one can at least say that, ifP vs. NP(or for that matter, theRiemann hypothesis,Goldbach’s conjecture, etc.) were proven independent of ZF, it would be an unprecedented development. [Scott Aaronson,, 2016].P vs. NP

Speaking of * Goldbach’s Conjecture*, the most talked about and the most intuitively correct statement in Number Theory that I know. In a publicity stunt, for two years there was a

**$1 million**prize by a publishing house for the

*proof of the conjecture*. Why just for the proof? I never heard of anyone not believing the conjecture. If I was the insurance underwriter for the prize (I bet they had one), I would allow them to use “for the proof or disproof” for a mere extra

**$100**in premium. For another

**$50**I would let them use “or independent of ZF” — it’s a free money, so why not? It’s such a pernicious idea of rewarding only one kind of research outcome!

Curiously, even for *Goldbach’s Conjecture*, there is a mild divergence of POVs on what the future holds. For example, Popper writes (twice in the same book!) that:

[On whether

Goldbach’s Conjectureis ‘demonstrable’] We don’t know: perhaps we may never know, and perhaps we can never know. [Karl Popper,Conjectures and Refutations, 1963]

Ugh. Perhaps. I suppose *anything *can happen… For example, our civilizations can “perhaps” die out in the next 200 years. But is that likely? Shouldn’t the gloomy past be a warning, not a prediction of the future? The only thing more outrageously pessimistic is this theological gem of a quote:

Not even God knows the number of permutations of 1000 avoiding the

1324 pattern. [Doron Zeilberger, quoted here, 2005]

Thanks, Doron! What a way to encourage everyone! Since we know from numerical estimates that this number is ≈ 3.7 × 10^{1017} (see this paper and this follow up), Zeilberger is suggesting that large pattern avoidance numbers are impossibly hard to compute *precisely*, already in the range of only about 1018 digits. I really hope he is proved wrong in his lifetime.

But I digress. What I mean to emphasize, is that there are many ways a problem can be resolved. Yet some outcomes are considered more valuable than others. Shouldn’t the research achievements be rewarded, not the desired outcome? Here is yet another colorful opinion on this:

Given a conjecture, the best thing is to prove it. The second best thing is to disprove it. The third best thing is to prove that it is not possible to disprove it, since it will tell you not to waste your time trying to disprove it. That’s what Gödel did for the Continuum Hypothesis. [Saharon Shelah,

Rutgers Univ. Colloqium, 2001]

#### Why do I care?

For one thing, disproving conjectures is part of what I do. Sometimes people are a little shy to unambiguously state them as formal conjectures, so they phrase them as *questions *or *open problems*, but then clarify that they believe the answer is positive. This is a distinction without a difference, or at least I don’t see any (maybe they are afraid of Sarnak’s wrath?) Regardless, proving their beliefs wrong is still what I do.

For example, here is my old bog post on my disproof of the *Noonan-Zeiberger Conjecture* (joint with Scott Garrabrant). And in this recent paper (joint with Danny Nguyen), we disprove in one big swoosh both *Barvinok’s Problem*, *Kannan’s Problem*, and *Woods Conjecture*. Just this year I disproved three conjectures:

- The
*Kirillov–Klyachko Conjecture*(2004) that the*reduced Kronecker coefficients*satisfy the saturation property (this paper, joint with Greta Panova). - The
*Brandolini et al. Conjecture*(2019) that concrete lattice polytopes can multitile the space (this paper, joint with Alexey Garber). *Kenyon’s Problem*(c. 2005) that every integral curve in**R**^{3}is a boundary of a PL surface comprised of unit triangles (this paper, joint with Alexey Glazyrin).

On top of that, just two months ago in this paper (joint with Han Lyu), we showed that the remarkable *independence heuristic* by I. J. Good for the number of *contingency tables*, fails badly even for nearly all uniform marginals. This is not exactly disproof of a conjecture, but it’s close, since the heuristic was introduced back in 1950 and continues to work well in practice.

In addition, I am currently working on disproving two more old conjectures which will remain unnamed until the time we actually resolve them (which might never happen, of course). In summary, I am deeply vested in disproving conjectures. The reasons why are somewhat complicated (see some of them below). But whatever my reasons, I demand and naively fully expect that my disproofs be treated on par with proofs, regardless whether this expectation bears any relation to reality.

#### My favorite disproofs and counterexamples:

There are many. Here are just a few, some famous and some not-so-famous, in historical order:

*Fermat‘s conjecture*(letter to Pascal, 1640) on primality of*Fermat numbers*, disproved by Euler (1747)*Tait’s conjecture*(1884) on hamiltonicity of graphs of simple 3-polytopes, disproved by W.T. Tutte (1946)*General Burnside Problem*(1902) on finiteness of periodic groups, resolved negatively by E.S. Golod (1964)*Keller’s conjecture*(1930) on tilings with unit hypercubes, disproved by Jeff Lagarias and Peter Shor (1992)*Borsuk’s Conjecture*(1932) on partitions of convex sets into parts of smaller diameter, disproved by Jeff Kahn and Gil Kalai (1993)*Hirsch Conjecture*(1957) on the diameter of graphs of convex polytopes, disproved by Paco Santos (2010)*Woods’s conjecture*(1972) on the covering radius of certain lattices, disproved by Oded Regev, Uri Shapira and Barak Weiss (2017)*Connes embedding problem*(1976), resolved negatively by Zhengfeng Ji, Anand Natarajan, Thomas Vidick, John Wright and Henry Yuen (2020)

In all these cases, the disproofs and counterexamples didn’t stop the research. On the contrary, they gave a push to further (sometimes numerous) developments in the area.

#### Why should you disprove conjectures?

There are three reasons, of different nature and importance.

**First**, disproving conjectures is * opportunistic*. As mentioned above, people seem to try proving much harder than they try disproving. This creates niches of opportunity for an open-minded mathematician.

**Second**, disproving conjectures is * beautiful*. Let me explain. Conjectures tend to be

*rigid*, as in “objects of the type

*pqr*satisfy property

*abc*.” People like me believe in the idea of “

*universality*“. Some might call it “

*completeness*” or even “

*Murphy’s law*“, but the general principle is always the same. Namely: it is not sufficient that one

*that all*

**wishes***pqr*satisfy

*abc*to actually believe in the implication; rather, there has to be a

*why*

**strong reason***abc*should hold. Barring that,

*pqr*can possibly be almost anything, so in particular

*non-abc*. While some would argue that

*non-abc*objects are “ugly” or at least “not as nice” as

*abc*, the idea of

*means that your objects can be of*

*universality**every color of the rainbow*— nice color, ugly color, startling color, quiet color, etc. That kind of palette has its own

*sense of beauty*, but it’s an acquired taste I suppose.

**Third**, disproving conjectures is * constructive*. It depends on the nature of the conjecture, of course, but one is often faced with necessity to

*construct*a counterexample. Think of this as an engineering problem of building some

*pqr*which at the same time is not

*abc*. Such construction, if at all possible, might be difficult, time consuming and computer assisted. But so what? What would you rather do: build a mile-high skyscraper (none exist yet) or prove that this is impossible? Curiously, in CS Theory both algorithms and (many) complexity results are constructive (you need gadgets). Even the GCT is partially constructive, although explaining that would take us awhile.

#### What should the institutions do?

If you are an *institution which awards prizes*, stop with the legal nonsense: “We award […] only for a publication of a proof in a top journal”. You need to set up a scientific committee anyway, since otherwise it’s hard to tell sometimes if someone deserves a prize. With mathematicians you can expect anything anyway. Some would post two arXiv preprints, give a few lectures and then stop answering emails. Others would publish only in a journal where they are Editor-in-Chief. It’s stranger than fiction, really.

What you should do is say in the official rules: “We have [**this much money**] and an independent scientific committee which will award any progress on [**this problem**] partially or in full as they see fit.” Then a disproof or an independence result will receive just as much as the proof (what’s done is done, what else are you going to do with the money?) This would also allow some flexibility for partial solutions. Say, somebody proves *Goldbach’s Conjecture* for integers > exp(exp(10^{100000})), way way beyond computational powers for the remaining integers to be checked. I would give this person at least 50% of the prize money, leaving the rest for future developments of possibly many people improving on the bound. However, under the old prize rules such person gets bupkes for their breakthrough.

#### What should the journals do?

In short, become more open to results of computational and experimental nature. If this sounds familiar, that’s because it’s a summary of* Zeilberger’s Opinions*, viewed charitably. He is correct on this. This includes publishing results of the type “Based on computational evidence we believe in the following *UVW *conjecture” or “We develop a new algorithm which confirms the *UVW* conjecture for n<13″. These are still contributions to mathematics, and the journals should learn to recognize them as such.

To put in context of our theme, it is clear that a lot more effort has been placed on proofs than on finding counterexamples. However, in many areas of mathematics there are no *small* counterexamples, so a heavy computational effort is crucial for any hope of finding one. Such work is not be as glamorous as traditional papers. But really, when it comes to standards, if a journal is willing to publish the study of something like the “*null graphs*“, the ship has sailed for you…

Let me give you a concrete example where a computational effort is indispensable. The curious *Lovász conjecture* states that every finite connected vertex-transitive graph contains a Hamiltonian path. This conjecture got to be false. It hits every red flag — there is really no reason why *pqr* = “vertex transitive” should imply *abc *= “Hamiltonian”. The best lower bound for the length of the longest (self-avoiding) path is only about square root of the number of vertices. In fact, even the original wording by Lovász shows he didn’t believe the conjecture is true (also, I asked him and he confirmed).

Unfortunately, proving that some potential counterexample is not Hamiltonian is computationally difficult. I once had an idea of one (a nice cubic Cayley graph on “only” 3600 vertices), but Bill Cook quickly found a Hamiltonian cycle dashing my hopes (it was kind of him to look into this problem). Maybe someday, when the TSP solvers are fast enough on much larger graphs, it will be time to return to this problem and thoroughly test it on large Cayley graphs. But say, despite long odds, I succeed and find a counterexample. Would a top journal publish such a paper?

#### Editor’s dilemma

There are three real criteria for evaluation a solution of an open problem by the journal:

- Is this an old, famous, or well-studied problem?
- Are the tools interesting or innovative enough to be helpful in future studies?
- Are the implications of the solution to other problems important enough?

Now let’s make a hypothetical experiment. Let’s say a paper is submitted to a top math journal which solves a famous open problem in Combinatorics. Further, let’s say somebody already proved it is equivalent to a major problem in TCS. This checks criteria 1 and 3. Until not long ago it would be rejected regardless, so let’s assume this is happening relatively recently.

Now imagine two parallel worlds, where in the first world the conjecture is *proved* on 2 pages using beautiful but elementary linear algebra, and in the second world the conjecture is *disproved* on a 2 page long summary of a detailed computational search. So in neither world we have much to satisfy criterion 2. Now, a quiz: in which world the paper will be published?

If you recognized that the first world is a story of Hao Huang‘s elegant proof of the *induced subgraphs of hypercubes conjecture*, which implies the *sensitivity conjecture*. The *Annals *published it, I am happy to learn, in a welcome break with the past. But unless we are talking about some 200 year old famous conjecture, I can’t imagine the *Annals* accepting a short computational paper in the second world. Indeed, it took a bit of a scandal to accept even the 400 year old *Kepler’s conjecture* which was ** proved **in a remarkable computational work.

Now think about this. Is any of that fair? Shouldn’t we do better as a community on this issue?

#### What do other people do?

Over the years I asked a number of people about the uncertainty created by the conjectures and what do they do about it. The answers surprised me. Here I am paraphrasing them:

** Some **were

*dumbfounded*: “What do you mean this conjecture could be false? It has to be true, otherwise nothing I am doing make much sense.”

** Others **were

*simplistic*: “It’s an important conjecture. Famous people said it’s true. It’s my job to prove it.”

** Third **were

*defensive*: “Do you really think this conjecture could be wrong? Why don’t you try to disprove it then? We’ll see who is right.”

** Fourth **were

*biblical*: “I tend to work 6 days a week towards the proof and one day towards the disproof.”

** Fifth **were

*practical*: “I work on the proof until I hit a wall. I use the idea of this obstacle to try constructing potential counterexamples. When I find an approach to discard such counterexamples, I try to generalize the approach to continue working on the proof. Continue until either side wins.”

If the last two seem sensible to you to, that’s because they are. However, I bet * fourth* are just grandstanding — no way they actually do that. The

*sound great when this is possible, but that’s exceedingly rare, in my opinion. We live in a technical age when proving new results often requires great deal of effort and technology. You likely have tools and intuition to work in only one direction. Why would you want to waste time working in another?*

**fifth**#### What should you do?

**First**, remember to *make conjectures*. Every time you write a paper, tell a story of what you proved. Then tell a story of what you wanted to prove but couldn’t. State it in the form of a conjecture. Don’t be afraid to be wrong, or be right but oversharing your ideas. It’s a downside, sure. But the upside is that your conjecture might prove very useful to others, especially young researchers. In might advance the area, or help you find a collaborator to resolve it.

**Second**, learn to *check your conjectures* computationally in many small cases. It’s important to give supporting evidence so that others take your conjectures seriously.

**Third**, learn to *make experiments*, explore the area computationally. That’s how you make new conjectures.

**Fourth**, *understand yourself*. Your skill, your tools. Your abilities like problem solving, absorbing information from the literature, or making bridges to other fields. Faced with a conjecture, use this knowledge to understand whether at least in principle you might be able to prove or disprove a conjecture.

**Fifth**, actively *look for collaborators*. Those who have skills, tools, or abilities you are missing. More importantly, they might have a different POV on the validity of the conjecture and how one might want to attack it. Argue with them and learn from them.

**Sixth**, *be brave* and *optimistic*! Whether you decide to prove, disprove a conjecture, or simply state a new conjecture, go for it! Ignore the judgements by the likes of Sarnak and Zeilberger. Trust me — they don’t really mean it.

## Take an interview!

We all agree that Math is a *human endeavor*, yet we know so preciously little about mathematicians as humans working in mathematics. Our papers tend to have preciously few quotable sentences outside of the dry mathematical context. In fact, most introductions are filled with passages of the form “X introduced the celebrated tool *pqr*, which over the next 20 years was refined by A, B and C, and most recently was used by D to prove *Z’s conjecture*“. It is such a weak tea to convey contributions of six people in one short sentence, yet we all do this nonetheless.

In my “*How to write a clear math paper*” article accompanying this blog post, I argue that at least the first paragraph or the first subsection of a long paper can be human and aimed at humans. That is the place where one has freedom to be eloquent, inspiring, congratulatory, prescient, revelatory and quotable. I still believe that, but now I have a new suggestion, see the title of this blog post.

#### The art of autobiographies

These days many great scientists remain active into very old age, and rarely want or have time to write an autobiography. Good for them, bad for us. Psychologically this is understandable — it feels a little *epitaphish*, so they would much rather have someone else do that. But then their real voice and honest thoughts on life and math are lost, and can never be recorded. There is blogging, of course, but that’s clearly not for everyone.

There are some notable exceptions to this, of course. When I was in High School, reading autobiographies of Richard Feynman, Stan Ulam and Norbert Wiener was a pure joy, a window into a new world. The autobiоgraphy by Sofya Kovalevskaya was short on mathematical stories, but was so well written I think I finished the whole thing in one sitting. G.H. Hardy’s “*Apology*” is written in different style, but clearly self-revealing; while I personally disagree with much of his general point, I can see why the book continues to be read and inspire passionate debates.

More recently, I read William Tutte, “*Graph Theory As I Have Known It*“, which is mostly mathematical, but with a lot of personal stories delivered in an authoritative voice. It’s a remarkable book, I can’t praise it enough. Another one of my favorites is Steven Krantz, “*Mathematical Apocrypha*” and its followup, which are written in the first person, in a pleasant light rumor mill style. Many stories in these near-autobiographies were a common knowledge decades ago (even if some were urban legends), but are often the only way for us to learn *now* how it was back then.

On the opposite end of the spectrum there is L.S. Pontryagin’s autobiography (in Russian), which is full of wild rumors, vile accusations, and banal antisemitism. The book is a giant self-own, yet I couldn’t stop myself from hate-reading the whole thing just so I could hear all these interesting old stories from horse’s mouth.

Lately, the autobiographies I’ve been reading are getting less and less personal, with little more than background blurbs about each paper. Here are those by George Lusztig and Richard Stanley. It’s an unusual genre, and I applaud the authors for taking time to write these. But these condensed CV-like auto-bios clearly leave a lot of room for stories and details.

#### Why an interview?

Because a skillful interviewer can help a mathematician reveal personal stories, mathematical and metamathematical beliefs, and even general views (including controversial ones). Basically, reveal the humanity of a person that otherwise remains guarded behind endless Definition-Lemma-Theorem constructions.

Another reason to interview a person is to **honor** her or his contributions to mathematics. In the aftermath of my previous blog post, I got a lot of contradictory push-back. Some would say “*I am shocked, shocked, to find that there is corruption going on. I have submitted to many invited issues, served as a guest editor for others and saw none of that! So you must be wrong, wrong, wrong*.” Obviously, I am combining several POVs, satirizing and paraphrasing for the effect.

Others would say “*Yes, you are right, some journals are not great so my junior coauthors do suffer, the refereeing is not always rigorous, the invited authors are often not selected very broadly, but what can I do? The only way I can imagine to honor a person is by a math article in an invited issue of a peer review journal, so we must continue this practice*” (same disclaimer as above). Yeah, ok the imaginary dude, that’s just self-serving with a pretense of being generous and self-sacrificing. (Yes, my straw man fighting skill are unparalleled).

In fact, there are many ways to honor a person. You can give a talk about that person’s contributions, write a survey or a biographical article, organize a celebratory conference, or if you don’t want to be bothered simply add a dedication in the beginning of the next article you publish. Or, better yet, **interview the honoree**. Obviously, do this some time soon, while this person is alive, and make sure to put the interview online for everyone to read or hear.

#### How to do an interview?

Oh, you know, via Zoom, for example. The technical aspects are really trivial these days. With permission, you can record the audio/video by pushing one button. The very same Zoom (or Apple, Google, Amazon, Microsoft, etc.) have good speech-to-text programs which will typeset the whole interview for you, modulo some light editing (especially of math terminology). Again, with a couple of clicks, you can publish the video or the audio on YouTube, the text on your own website or any social media. Done. Really, it’s *that* easy!

#### Examples

I have many favorites, in fact. One superb **video collection** is done by the Simons Institute. I already blogged here about terrific interviews with László Lovász and Endre Szemerédi. The interviewer for both is Avi Wigderson, who is obviously extremely knowledgeable of the subject. He asked many pointed and interesting questions, yet leaving the interviewees plenty of space to develop and expand on their their answers. The videos are then well edited and broken into short watchable pieces.

Another interesting collection of video interviews is made by CIRM (in both English and French). See also general video collections, some of which have rather extensive and professionally made interviews with a number of notable mathematicians and scientists. Let me single out the *Web of Stories*, which include lengthy fascinating interviews with Michael Atiyah, Freeman Dyson, Don Knuth, Marvin Minsky, and many others.

I already wrote about how to watch a math video talk (some advice may be dated). Here it’s even easier. At the time of the pandemic, when you are Zoom fatigued — put these on your big screen TV and watch them as documentaries with as much or as little attention as you like. I bet you will find them more enlightening than the news, Netflix or other alternatives.

Authorized biography **books **are less frequent, obviously, but they do exist. One notable recent example is “*Genius At Play: The Curious Mind of John Horton Conway*” by Siobhan Roberts which is based on many direct conversations. Let me also single out perhaps lesser known “*Creative Minds, Charmed Lives*” by Yu Kiang Leong, which has a number of interesting interviews with excellent mathematicians, many of the them not on other lists. For example, on my “*What is Combinatorics*” page, I quote extensively from his interview with Béla Bollobás, but in fact the whole interview is worth reading.

Finally, there is a truly remarkable collection of **audio **interviews by Eugene Dynkin with leading mathematicians of his era, spanning from 1970s to 2010s (some in English, some in Russian). The collection was digitized using Flash which died about five years ago, rendering the collection unusable. When preparing this post I was going to use this example as a cautionary tale, but to my surprise someone made it possible to download them in .mp3. Enjoy! Listening to these conversations is just delightful.

#### Final thoughts

Remember, you don’t have to be a professional interviewer to do a good job. Consider two most recent interviews with Noga Alon and Richard Stanley by Toufik Mansour, both published at *ECA*. By employing a simple trick of asking the same well prepared questions, he allows the reader to compare and contrast the answers, and make their own judgement on which ones they like or agree with the most. Some answers are also quite revealing, e.g. Stanley saying he occasionally thinks about the RH (who knew?), or Alon’s strong belief that “*mathematics should be considered as one unit*” (i.e. without the area divisions). The problems they consider to be important are also rather telling.

Let me mention that in the digital era, even the amateur long forgotten interviews can later be found and proved useful. For example, I concluded my “*History of Catalan numbers*” with a quote from an obscure Richard Stanley’s interview to the MIT undergraduate newspaper. There, he was discussing the origins of his Catalan numbers exercise which is now a book. Richard later wrote to me in astonishment as he actually completely forgot he gave that interview.

**So, happy watching, listening, and reading all the interviews! Hope you take some interviews yourself for all of us to enjoy!**

**P.S.** (Added Dec 3, 2020) At my urging, Bruce Rothschild has typed up a brief “*History of Combinatorics at UCLA*“. I only added hyperlinks to it, to clarify the personalities Bruce is talking about (thus, all link mistakes are mine).

**P.P.S.** (Added Feb 6, 2021) At my request, the editors of *ECA* clarified their interview process (as of today, they have posted nine of them). Their interviews are conducted over email and are essentially replies to the nearly identical sets of questions. The responses are edited for clarity and undergo several rounds of approval by the interviewee. This practice is short of what one would traditionally describe as a *journalistic interview* (e.g., there are no uncomfortable questions), and is more akin to *writing a puff piece*. Still, we strongly support this initiative by the *ECA* as the first systematic effort to put combinatorialists on record. Hopefully, with passage of time others types of interviews will also emerge from various sources.

## How Combinatorics became legitimate (according to László Lovász and Endre Szemerédi)

* Simons Foundation* has a series of fantastic interviews with leading mathematicians (ht Federico Ardila). Let me single out the interviews with László Lovász and Endre Szemerédi. Avi Wigderson asked both of them about the history of combinatorics and how it came into prominence. Watch parts 8-9 in Lovász’s interview and 10-11 in Szemerédi’s interview to hear their fascinating answers.

**P.S.** See also my old blog posts on what is combinatorics, how it became legitimate and how to watch math videos.

## Some good news

Two of my former Ph.D. students won major prizes recently — **Matjaž Konvalinka** and **Danny Nguyen**. Matjaž is an Associate Professor at University of Ljubljana, Danny is a Lewis Research Assistant Professor at University of Michigan, Ann Arbor. **Congratulations to both of them**!

(1) The * 2019 Robbins Prize *is awarded to

**Roger Behrend**,

**Ilse Fischer**and

**Matjaž Konvalinka**for their paper “

*Diagonally and antidiagonally symmetric alternating sign matrices of odd order*”. The

*Robbins Prize*is given in Combinatorics and related areas of interest is named after the late David P. Robbins and is given once every 3 years by AMS and MAA.

In many ways, this paper completes the long project of enumerating *alternating sign matrices* (ASMs) initiated by William Mills, David Robbins, and Howard Rumsey in the early 1980s. The original #ASM(*n*)=#TSSCPP(*n*) conjecture follows from Andrews’s proof of the conjectured product formula for #TSSCPP(*n*), and Zeilberger’s 84 page computer assisted proof of the the same conjectured product formula for #ASM(*n*). This led to a long series of remarkable developments which include Kuperberg’s proof using the *Izergin-Korepin determinant* for the six vertex model, the Cantini–Sportiello proof of the *Razumov-Stroganov conjecture,* and a recent self-contained determinantal proof for the number of ASMs by Fischer. Bressoud’s book (and this talk, slides) is a good introduction. But the full story is yet to be written.

(2) The ** 2018 Sacks Prize** is awarded to

**Danny Nguyen**for his UCLA Ph.D. dissertation on the complexity of short formulas in

*Presburger Arithmetic*(PA) and many related works (some joint with me, some with others). See also the UCLA announcement. The S

*acks Prize*is given by the international

**Association for Symbolic Logic**for “the most outstanding doctoral dissertation in mathematical logic“. It is sometimes shared between two awardees, and sometimes not given at all. This year Danny is the sole winner of the prize.

Danny’s dissertation is a compilation of eight (!) papers Danny wrote during his graduate studies, all on the same or closely related subject. These papers advance and mostly finish off the long program of understanding the boundary of what’s feasible in PA. The most important of these is our joint **FOCS paper** which basically says that *Integer Programming* and *Parametric Integer Programming* is all that’s left in **P**, while all longer formulas are **NP-hard**. See Featured MathSciNet Review by Sasha Barvinok and an overlapping blog post by Gil Kalai discussing these results. See also Danny’s FOCS talk video and my MSRI talk video presenting this work.

## Just combinatorics matters

I would really like everyone to know that every time you say or write that something is “just combinatorics” somebody rolls his eyes. Guess who?

Here is a short collection of “just combinatorics” quotes. It’s a followup on my “What is Combinatorics?” quotes page inspired by the “What is Combinatorics?” blog post.

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