I will tell you a story about the Reciprocity Law. After my thesis, I had the idea to define $L$-series for non-abelian extensions. But for them to agree with the $L$-series for abelian extensions, a certain isomorphism had to be true. I could show it implied all the standard reciprocity laws. So I called it the General Reciprocity Law and tried to prove it but couldn’t, even after many tries. Then I showed it to the other number theorists, but they all laughed at it, and I remember Hasse in particular telling me it couldn’t possibly be true.

Still, I kept at it, but nothing I tried worked. Not a week went by — for three years! — that I did not try to prove the Reciprocity Law. It was discouraging, and meanwhile I turned to other things. Then one afternoon I had nothing …

One of the pieces of advice I constantly give to young students preparing for math contests is that they should probably do harder problems. But perhaps I don’t preach this zealously enough for them to listen, so here’s a concrete reason (with actual math!) why I give this advice.

1. The AIME and USAMO

In the USA many students who seriously prepare for math contests eventually qualify for an exam called the AIME (American Invitational Math Exam). This is a 3-hour exam with 15 short-answer problems; the median score is maybe about 5 problems.

Correctly solving maybe 10 of the problems qualifies for the much more difficult USAMO. This national olympiad is much more daunting, with six proof-based problems given over nine hours. It is not uncommon for olympiad contestants to not solve a single problem (this certainly happened to me a fair share of times!).

You’ll …

As part of the 18.099 Discrete Analysis reading group at MIT, I presented section 4.7 of Tao-Vu’s Additive Combinatorics textbook. Here were the notes I used for the second half of my presentation.

1. Synopsis

We aim to prove the following result.

Theorem 1 (Bourgain)

Assume $N \ge 2$ is prime and $A, B \subseteq Z = \mathbb Z_N$. Assume that $$\delta \gg \sqrt{\frac{(\log \log N)^3}{\log N}}$$ is such that $\min\left\{ \mathbf P_ZA, \mathbf P_ZB \right\} \ge \delta$. Then $A+B$ contains a proper arithmetic progression of length at least $$\exp (\mathrm{C\; \dots }$$

Some time ago I was reading the 18.785 analytic NT notes to try and figure out some sections of Davenport that I couldn’t understand. These notes looked nice enough that I decided I should probably print them out, But much to my annoyance, I found that almost all the top margins were too tiny, and the bottom margins too big. (I say “almost all” since the lectures 19 and 24 (Bombieri proof and elliptic curves) were totally fine, for inexplicable reasons).

Thus, instead of reading Davenport like I told myself to, I ended up learning enough GhostScript flags to write the following short script, which I’m going to share today so that other people can find better things to do with their time.

#!/bin/bash
for file in $@
  do
    echo "Shifting $file ..."
    gs \
      -sDEVICE=pdfwrite \
      -o shifted-$file \
 -dPDFSETTINGS=/prepress \
 -c "<</PageOffset [0 -36]>> setpagedevice" \
 -f $file …

As part of the 18.099 discrete analysis reading group at MIT, I presented section 4.7 of Tao-Vu’s Additive Combinatorics textbook. Here were the notes I used for the first part of my presentation.

1. Synopsis

In the previous few lectures we’ve worked hard at developing the notion of characters, Bohr sets, spectrums. Today we put this all together to prove some Szemerédi-style results on arithmetic progressions of $\mathbb Z_N$.

Recall that Szemerédi’s Theorem states that:

Theorem 1 (Szemerédi)

Let $k \ge 3$ be an integer. Then for sufficiently large $N$, any subset of $\{1, \dots, N\}$ with density at least $$\frac{1}{(\log \log N)^{2^{-2^k+9}}}$$ contains a length $\mathrm{k\; \dots }$

Happy Pi Day! I have an economics midterm on Wednesday, so here is my attempt at studying.

1. Mechanisms

The idea is as follows.

• We have $N$ people and a seller who wants to auction off a power drill.
• The $i$-th person has a private value of at most $\$1000$ on the power drill. We denote it by $x_i \in [0,1000]$.
• However, everyone knows the $x_i$ are distributed according to some measure $\mu_i$ supported on $[0, 1000]$. (Let’s say a Radon measure, but I don’t especially care). Tacitly we assume $\mu_i([0,1000]) = 1$.

Definition 1. Consider a game $M$ played as follows:

• Each player $i=1,\dots ,\mathrm{N\; \dots }$

These notes are from the February 23, 2016 lecture of 18.757, Representations of Lie Algebras, taught by Laura Rider.

Fix a field $k$ and let $G$ be a finite group. In this post we will show that one can reconstruct the group $G$ from the monoidal category of $k[G]$-modules (i.e. its $G$-representations).

1. Hopf algebras

We won’t do anything with Hopf algebras per se, but it will be convenient to have the language.

Recall that an associative $k$-algebra is a $k$-vector space $A$ equipped with a map $m : A \otimes A \rightarrow A$ and $i : k \hookrightarrow A$ (unit), satisfying some certain axioms.

Then a $k$-coalgebra is …

[EDIT 2018/03/05: This description seems significantly less accurate to me now than it did a few years ago, both because my views/values have changed substantially, and because SPARC has changed direction substantially since I attended as a junior counselor in 2015. I’ll leave it here as a reference, but should be taken with a grain of salt.]

I often get asked about what I learned from the SPARC summer camp. This is hard to describe and I never manage to give as a good of an answer as I want, so I want to take the time to write down something concrete now. For context: I attended SPARC in 2013 and 2014 and again as a counselor in 2015, so this post is long overdue (but better late than never).

(For those of you still in high school: applications for 2016 are now open, due March …

Occasionally I am approached by parents who ask me if I am available to teach their child in olympiad math. This is flattering enough that I’ve even said yes a few times, but I’m always confused why the question is “can you tutor my child?” instead of “do you think tutoring would help, and if so, can you tutor my child?”.

Here are my thoughts on the latter question.

Charging by Salt

I’m going to start by clearing up the big misconception which inspired the title of this post.

The way tutoring works is very roughly like the following: I meet with the student once every week, with custom-made materials. Then I give them some practice problems to work on (“homework”), which I also grade. I throw in some mock olympiads. I strongly encourage my students to email me with questions as they come up. Rinse and …

I know some friends who are fantastic at synthetic geometry. I can give them any problem and they’ll come up with an incredibly impressive synthetic solution. I also have some friends who are very bad at synthetic geometry, but have such good fortitude at computations that they can get away with using Cartesian coordinates for everything.

I don’t consider myself either of these types; I don’t have much ingenuity when it comes to my solutions, and I’m actually quite clumsy when it comes to long calculations. But nonetheless I have a high success rate with olympiad geometry problems. Not only that, but my solutions are often very algorithmic, in the sense that any well-trained student should be able to come up with this solution.

In this article I try to describe how I come up which such solutions.

1. The Three Reductions

Very roughly, there are …