Consider $n$ observed data points $(x_1,y_1),\dots, (x_n, y_n)$. We think they might satisfy a linear model $y = ax + b$. Finding the coefficients $a$ and $b$ is called linear regression, and the most typical way to find them is the method of least squares: that is, we find $a$ and $b$ that minimize the […]
Applying for jobs after a PhD and my postdoc was one the weirdest things I ever did. I haven't written too much about it before, but because it is so bewildering I thought I'd give out some stats on how my application process went. The most obvious statistic is the number of jobs to which […]
I've talked a lot about von Neumann regular rings on this blog, so I thought I'd write an informal short survey on them, collecting some facts we've already seen and many new ones. It should give you an idea of what von Neumann regular rings are. Most of the facts that I did not explicitly […]
We say that an associative ring $A$ is von Neumann regular if for every $a\in A$ there exists a $x\in A$ such that $axa = a$. That is a rather strange condition, isn't it? But, you can think of $x$ as a pseudoinverse to $a$. This weakening of inverses has a homological counterpart: if every […]
On July 10, 2019, the University of California gave up its access to Elsevier journals. According to Elsevier, The contract ended in December 2018. Since then, while working to find a solution, we have continued to provide access without payment to University of California campuses. Unfortunately, we've been unable to come to an agreement. The […]
A proof assistant is a computer program that takes as input a proof in a formal language and outputs true if and only if the proof is a valid proof in a formal system. An example would be first-order logic with its inference rules. Proof assistants are supposed to tell you whether your proof is […]
Take a number written in decimal, like $25$. Take the sum of squares of its digits: $2^2 + 5^2 = 29$. Can you ever get the number you started with? In fact, no positive natural number greater than one is the sum of squares of its decimal digits. However, 75 is pretty close: $7^2 + […]
This is a short list of books to get you started on learning automorphic representations. Before I talk about them, I will first define automorphic representation, which will take a few paragraphs. To start, we need an affine algebraic $F$-group scheme $G$ where $F$ is a number field or function field. We let $\A_F$ be […]
A perfect number is a positive integer $n$ such that $n$ is the sum of its proper divisors. For example $6 = 1 + 2 + 3$. The symbol $\sigma(n)$ is usally used for the sum of all the divisors of a positive integer $n$, so that a number is perfect if and only if […]
You can ask lots of questions about primes. After posting 50 facts about primes, I couldn't resist making another graph. In this one, the x-axis is $n$ and the y-axis is the number of primes up to $n$ that contain a given decimal digit (written in decimal, of course). I've plotted all of these on […]