May 25, 2023 by Adam Sheffer

A Guide for Students Presenting their First Talk

I wrote a guide for undergraduate students who prepare to present their first math talk, for participants of the Polymath Jr Program and the NYC Discrete Math REU. I wanted to also share the guide here for two reasons:

Perhaps it would be useful to others.
These are just my personal opinions and I would love to hear others!

Click here for the pdf file.

To the many friends who helped preparing this, THANK YOU! (credits in the pdf).

September 18, 2022 by Adam Sheffer

New CS Tenure Track Positions, New program in Manhattan

Do you happen to know anyone who is looking for a CS Tenure Track position in Manhattan? Someone who might be interested in being part of a new CS program and help decide the directions it will grow into? Please share this with them: https://geometrynyc.wixsite.com/csjobs.

March 3, 2022 by Adam Sheffer

New Book: Polynomial Methods and Incidence Theory

This month, my new book “Polynomial Methods and Incidence Geometry” is coming out. You can order it and find more information on the website of Cambridge University Press.

This book aims to be an accessible introduction to the new polynomial methods and to incidence theory. For that reason, the book includes many examples, warm-up proofs, figures, and intuitive ways of thinking about tricky ideas. Many techniques are presented gradually and in detail.

The polynomial methods that the book studies include Dvir’s Finite Field Kakeya proof, Guth and Katz’s distinct distances proof, the proof of the Cap Set Conjecture, the proof of the joints theorem, and a lot more. Each topic includes exercises, main open problems, and references for digging deeper.

Please ask your university to order it!

November 3, 2021 by Adam Sheffer

CS Tenure Track Positions at CUNY

Interested in a computer science position in Manhattan? Apply to our positions here!

We are starting to form a computer science program at CUNY’s Baruch College. Joining us at the beginning of this process will give you a chance to influence how computer science will look like at our college: the research and teaching directions that it would focus on, the type of faculty that will be hired, and so on. We are accepting applications from all areas of computer science.

Yes, it is weird that we did not have computer science earlier. Traditionally, Baruch is CUNY’s business school, so the focus of computer classes has been on information systems rather than on computer science. However, recent trends in business have increased student demand for classes in computer science even among business majors.

The effort to create a computer science program is based at the mathematics department, in collaboration with Computer Information Systems, which is part of our business school. We already have a few people who work in computer science, such as Guy Moshkovitz and myself. We hope to hire several more computer scientists over the next few years.

We hope to have a diverse group of computer scientists. Women and underrepresented minorities are encouraged to apply!

For more information, you are welcome to contact me.

July 23, 2021 by Adam Sheffer

A Basic Question About Multiplicative Energy

As part of some research project, I got to a basic question about multiplicative energy. Embarrassingly , I wasn’t able to get any non-trivial bound for it. Here is the problem. Any information about it would be highly appreciated.

Problem. Let $A=\{1,2,3,\ldots,n^{1/2}\}$ . Let $B$ be a set of $n$ real numbers. How large can the multiplicative energy $E^\times(A,B)$ be?

Can we prove that $E^\times(A,B)$ must be asymptotically smaller than $n^{2}$ ?

Is there a $B$ for which $E^\times(A,B)$ is asymptotically larger than $n^{3/2}$ ?

Both questions are about improving the exponent of $n$ . Sub-polynomial improvements would be less interesting. Multiplicative energy means

$E^\times(A,B) = |\{(a,a',b,b')\in A^2\times B^2\ :\ ab=a'b'\}|$

Any ideas?

Isn’t this supposed to be an easy problem?!

A variant. Still here and want to read more? We can also consider the case when $A$ is larger. Let $A=\{1,2,3,\ldots,n^{\alpha}\}$ , where $1/2 < \alpha <1$ .

In this case, a simple argument leads to an upper bound that is better than the trivial one. Let $r_A(x) = |\{(a,a')\in A^2:\ a/a'=x \}|$ . The Cauchy-Schwarz inequality implies that

$E^\times(A,B) = |\{(a,a',b,b')\in A^2\times B^2\ :\ a/a'=b'/b\}|$

$= \sum_{x\in A/A} r_A(x) \cdot r_B(x) \le \sqrt{\sum_{x}r_A(x)^2} \cdot \sqrt{\sum_x r_B(x)^2}$

$=\sqrt{E^\times(A) \cdot E^\times(B)}$

It is not difficult to show that $E^\times(A)=\Theta(n^{2\alpha})$ , up to sub-polynomial factors. This implies that $E^\times(A,B)=O(n^{3/2+\alpha})$ , up to sub-polynomial factors. In this case, the trivial upper bound is $E^\times(A,B)=O(n^{1+2\alpha})$ .

The above still leaves a large gap between the lower bound $O(n^{1+\alpha})$ and the upper bound $E^\times(A,B)=O(n^{3/2+\alpha})$ , up to sub-polynomial factors.

Any ideas?

March 13, 2021 by Adam Sheffer

The 2021 Polymath Jr Program

Summer Opportunity: Polymath Jr Research Experience for Undergraduates. The goal of this remote program is to provide opportunities to undergraduates who wish to explore research mathematics. The program consists of research projects on a wide variety of mathematical topics. Each project is guided by an active researcher with experience in undergraduate mentoring. All undergraduates who have experience with writing mathematical proofs are eligible. Part-time participation is also allowed; preference is given to students who will not have an undergraduate degree by July 2021.

The program will run from June 21st to August 15th, 2021. For more details, see https://geometrynyc.wixsite.com/polymathreu and apply by early April at https://www.mathprograms.org/db; for additional questions contact adam.sheffer@baruch.cuny.edu.

December 30, 2020 by Adam Sheffer

19th Century Precursors of Polynomial Methods

These days, I’m spending a lot of time revising my book about polynomial methods and incidences (an older draft is available here). This led me to the following conclusion: I am not sure what classifies a proof as a polynomial method. A Wikipedia page states

“… the polynomial method is an algebraic approach to combinatorics problems that involves capturing some combinatorial structure using polynomials and proceeding to argue about their algebraic properties.”

I like this statement a lot, but I still don’t know whether some arguments should be considered as polynomial methods or not. There’s probably no good answer. Still, in this post, I want go over a case that I find interesting: If you have an inclusive view, you could claim that polynomial methods started in the 19th century. Anurag Bishnoi made a wonderful list of early polynomial method results, but it only goes back to 1975.

We can consider results such as the Monge-Cayley-Salmon theorem. However, for simplicity, let’s stick to shorter and more elementary proofs. Pascal’s theorem is from 1640. Two centuries later, Plücker provided an alternative proof for this theorem, which is based on studying basic properties of polynomials. For points ${a,b\in {\mathbb R}^2}$ , we denote the line incident to $a$ and $b$ as $\overline{ab}$ . For a polynomial $f$ , we denote as ${\bf V}(f)$ the variety defined by $f$ (that is, the set of points on which $f$ vanishes).

Pascal’s Theorem. Let $\gamma\subset {\mathbb R}^2$ be an ellipse, parabola, or hyperbola. Consider six distinct points $a,b,c,d,e,f\in \gamma$ . Then the three points $\overline{ab}\cap \overline{de}$ , $\overline{cd}\cap \overline{af}$ , and $\overline{ef}\cap \overline{bc}$ are collinear.

Proof. Note that $\gamma$ is an irreducible curve of degree 2. Let $h\in {\mathbb R}[x,y]$ be a degree 2 polynomial that satisfies $\gamma = {\bf V}(h)$ . We define two more curves:

$L_1 = \overline{ab}\cup \overline{cd} \cup \overline{ef},$

$L_2 = \overline{de}\cup \overline{af} \cup \overline{bc}.$

Both $L_1$ and $L_2$ are curves of degree 3, since each can be defined as the product of three linear equations. Let $g_1,g_2\in {\mathbb R}[x,y]$ be degree 3 polynomials that satisfy $L_1 = {\bf V}(g_1)$ and $L_2 = {\bf V}(g_2)$ . Let $p$ be an arbitrary point of $\gamma$ that is not one of $a,b,c,d,e,f$ . There exists $\alpha\in {\mathbb R}$ such that the polynomial $g = g_1 + \alpha g_2$ vanishes on $p$ . Indeed, one can think of $g(p)$ as a linear polynomial in $\alpha$ , and set $\alpha$ to be the unique root of this polynomial. We note that $g$ is of degree 3 and vanishes on $a,b,c,d,e,f,p$ .

By Bézout’s theorem, either $g$ and $h$ have a common factor or

$|{\bf V}(g)\cap {\bf V}(h)| \le \deg g \cdot \deg h = 3 \cdot 2 = 6.$

Since both $g$ and $h$ vanish on the seven points $a,b,c,d,e,f,p$ , we get that the two polynomials have a common factor. Since $h$ is irreducible, $h$ is a factor of $g$ . This implies that ${\bf V}(g)$ is the union of $\gamma$ and a line $\ell$ . Since $\gamma$ cannot contain the three intersection points $\overline{ab}\cap \overline{de}$ , $\overline{cd}\cap \overline{af}$ , and $\overline{ef}\cap \overline{bc}$ , these points are contained in $\ell$ . That is, the three intersection points are collinear. $\Box$

Plücker’s proof is a simple application of Bézout’s theorem. Should it be considered as a polynomial method? Purists might argue that polynomial proofs should involve asymptotically large configurations. That is, that the problem should involve n objects and a polynomial of degree that depends on $n$ . But this is not the case for some recent results. For example, people seem to consider the following proof as a polynomial method. It is also called a polynomial method in Larry Guth’s book about polynomial methods.

A regulus is the union of all lines in ${\mathbb R}^3$ that intersect three pairwise-skew lines $\ell_1,\ell_2,\ell_3$ . That is, no two lines of $\ell_1,\ell_2,\ell_3$ are parallel and no two intersect. One example of a regulus is the hyperbolic paraboloid ${\bf V}(z-xy)$ , depicted in the figure below. This surface is the union of all lines of the form ${\bf V}(x-c,z-cy)$ where $c\in {\mathbb R}$ . It is also the union of all lines of the form ${\bf V}(y-c,z-cx)$ . The lines in each of these families are pairwise-skew. We obtain ${\bf V}(z-xy)$ by taking the union of all lines that intersect three lines of the form ${\bf V}(y-c,z-cx)$ . This is the family of lines of the form ${\bf V}(x-c,z-cy)$ . Similarly, we obtain ${\bf V}(z-xy)$ by taking the union of all lines that intersect three lines of the form ${\bf V}(x-c,z-cy)$ .

Lemma. Every regulus in ${\mathbb R}^3$ is contained in an irreducible variety that is defined by a polynomial of degree two.

Proof. Consider a regulus $S\subset{\mathbb R}^3$ that is defined by the three pairwise-skew lines $\ell_1,\ell_2$ , and $\ell_3$ . Let ${\mathcal P}$ be a set of nine points that is obtained by arbitrarily choosing three points from each of $\ell_1,\ell_2$ , and $\ell_3$ . Since $\binom{3+2}{3} = 10 > 9$ , a standard polynomial parameter counting implies that there exists a nonzero $f\in {\mathbb R}[x,y,z]$ of degree at most two that vanishes on ${\mathcal P}$ . We apply Bézout’s theorem in a plane that contains $\ell_1$ and is not contained in ${\bf V}(f)$ . That is, we consider a linear bivariate polynomial that defines $\ell_1$ and a polynomial that defines the intersection of ${\bf V}(f)$ with the plane. Since $\ell_1$ intersects ${\bf V}(f)$ in at least three points, Bézout’s theorem implies that $\ell_1 \subset {\bf V}(f)$ . For the same reason, we also have that $\ell_2,\ell_3\subset {\bf V}(f)$ . If ${\bf V}(f)$ is a plane or a union of two planes, then at least two of $\ell_1,\ell_2,\ell_3$ are contained in a common plane. This would contradict the assumption that the three lines are pairwise-skew. We conclude that ${\bf V}(f)$ is an irreducible variety of degree two.

Consider a line $\ell'$ that intersects all three lines $\ell_1,\ell_2,\ell_3$ . The three intersection points are distinct since $\ell_1,\ell_2,\ell_3$ are disjoint. Since $\ell'$ intersects ${\bf V}(f)$ in at least three points, Bézout’s theorem implies that $\ell' \subset {\bf V}(f)$ . Since $S$ is the union of all lines that intersect $\ell_1,\ell_2,\ell_3$ , we get that $S\subseteq {\bf V}(f)$ .

So, can we start saying that polynomial methods go back to the 1840’s?

September 19, 2020 by Adam Sheffer

Combinatorial Journals are Changing

I used to ask most combinatorialists I met for their opinion about the level of various journals. With this feedback, I compiled a rough journal ranking for combinatorics papers (for personal use). This was a very educational experience for me as a new combinatorialist. I learned that different people have rather different opinions. For example, people tend to have a higher opinion of the journals of their own combinatorial sub-field.

However, people’s opinions did not vary much with respect to the very few top combinatorics journals. In particular, I don’t recall a single person who didn’t agree with Journal of Combinatorial Theory Series A being one of those (and similarly Combinatorica). This makes the following announcement quite significant:

“The majority of the JCT A editorial boards have recently notified Elsevier that they will not be renewing their contracts, and will resign after December 2020”

(Thank you Pablo Soberón for telling me about this.) The reason for this development is JCTA belonging to Elsevier. A new substitute journal has been created: Combinatorial Theory. This journal is “mathematician-owned, and fully open access, with no charges for authors or readers.” It already has a very impressive editorial board.

It would be interesting to see what happens next. So far not a lot of information can be found online. Are any combinatorialists plan to stand by JCTA? What about Elsevier’s top graph theory journal: Journal of Combinatorial Theory Series B?

There is one thing I really like about JCTA, and very much hope that Combinatorial Theory would adopt. Out of the stronger combinatorics journals, JCTA seems to be the one most open to all different types of combinatorics. From what I hear and see, all others tend to be at least somewhat biased towards some sub-fields. I’m told that JCTA originally focused on enumerative combinatorics, but this seems to be far from the case in recent years.

If you have more information about these developments, I would be very happy to hear!

September 4, 2020 by Adam Sheffer

Mathematics for Human Flourishing

I read a lot of “popular math” books. I also wrote about some in past posts. But I’ve never read a book similar to Mathematics for Human Flourishing by Francis Su. I already knew the math presented in this book and almost all other topics covered. I even knew some of Su’s personal stories from his past writings. But somehow that did not matter. I got more out of this book than from many other books whose topics were completely new to me.

So what is this unusual book about? Quoting from the first paragraph:

“… this is a book that grounds mathematics in what it means to be a human being and to live a more fully human life.”

Different people are likely to describe the main topic of the book in different ways. One might be “How one can live a healthy and fulfilling life in mathematics.” Another might be “How to do good in the world through mathematics (and not by solving a millennium prize problem).” There are likely to be many other different descriptions. But hopefully you get a vague idea.

To emphasize his main ideas, Su includes several “subplots” that develop throughout the book. One subplot is Su’s relationship with Christopher Jackson. Jackson is serving a 32-year prison sentence for a string of armed robberies. While in prison, he started reading basic math books, gradually moving to more advanced topics (under Su’s guidance). The correspondence between Jackson and Su exposes a fascinating story of how one finds their place in the world through math.

Another subplot is Su’s personal life. Su shares highly personal stories. Some stories are about his struggles with math and others unrelated to math. At least once I was so shocked by what Su was sharing that I had to stop reading for a few minutes.

A large portion of the book describes the mathematical world to people who are unfamiliar with it. This starts with the basic step of explaining that math is not about plugging numbers into formulas. It develops all the way to advanced topics such as how mentoring works. Su does excellent work explaining these ideas to non-mathematicians, in a fun and clear way. When describing ideas such as searching for patterns, real mathematical examples are provided. I would definitely recommend this book to family and non-math friends who wish to know more about what I do.

I would also recommend this book to students. While discussing how the math world works, it raises various issues that cause anxiety and distance people away from math. When working with students, I am going to steal some sentences, such as

“A popular myth of doing math is that either you see the answer or you don’t. In reality, you demonstrate mathematical power by assembling possible strategies and trying them to see if they will work.”

Since I know how the math world works, the mathematics explained in the book, and even some of Su’s personal stories, how could I get much out of this book? It got me to think about what exactly I like in math and in my work. It helped me see more clearly what my future goals are. It got me to think about how to better handle various tricky interactions with students and mentees. And more. I cannot say that about many books.

If your goal is to learn more math, do not read this book. If you are cynical about topics such as getting students less anxious about math, do not read this book. Read it if you are a mathematician who wants to rethink your attitude towards math and teaching. Or to get others more excited about math. Read it if you are a student who might have some anxiety about math or just want to know more about the math world. Recommend it to non-mathematicians who want to know what you are doing with your life.

August 5, 2020 by Adam Sheffer

Mind-Benders for the Quarantined

Peter Winkler is a world expert on mathematical puzzles (he is also an excellent researcher and this year’s resident mathematician of the MoMath). I just learned about two things that he is currently up to.

Winkler is running Mind-Benders for the Quarantined. After signing up to this free service, you receive a mathematical puzzle every Sunday, by email. You then get a subtle hint on Tuesday, a big hint on Thursday, and the solution on Saturday.

dartmouth-news-peter-winkler-810_455_0

Winkler’s new book should be out around the end of 2020. While he already published wonderful book collections of mathematical puzzles in the past, this one is meant to be a special collection of the world’s top puzzles. For example, the new book would also include the best puzzles from Winkler’s past books.

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Some Plane Truths

Discrete geometry and other typos