# Welcome to "Today I learned", a blog aboout math

Epsilons add up! I want to write about learning one thing every day. Could be big, could be small, but I just want to learn one thing every day.

• ### Two definitions of a variety

This semester I am participating in a reading group that is reading Fulton’s textbook on Toric Varieties. My motivation is to understand recent developments in matroid theory better, but in the process I am finding a need to shore up some basic geometry knowledge. Abstractly I know there are two definitions of an algebraic variety but I couldn’t remember the “coordinate free” one. My mathematical sister Sasha Pevzner showed me a way to remember it and why it makes sense in the context of the first one. This post is all about that.

• ### Two realizations of a matroid

A recent perspective on the study of matroids has been to understand them geometrically. Historically, a matroid has been considered to be realizable (or representable) if it can be written down as a finite set of vectors. The more modern approach views a realization as a vector subspace, thus opening the door for the use of tools from algebraic geometry, which are then extended to matroids which are not realizable.

• ### Log concavity implies unimodality

I have been to many talks where the speaker says something like “its not hard to see that log concavity (with no internal zeroes) implies unimodality,” but I have never seen the proof of this statement written down. So here it is, inspired by a recent talk by Petter Brändén.

• ### Parents, amirite?

I’m fortunate to have the support of the Google Summer of Code this summer to contribute to the SageMath (more commonly called Sage) mathematical software. To me one of the most confusing aspects of doing development in Sage is the “Parent/Element” framework. The goal of this post is to explain some details in the Parent/Element framework via a minimal implementation of some of the code I’m working on.

• ### Stirling numbers of the first kind

The Stirling numbers of the first kind, which I’ll write as $$c(n,k)$$, are the number of of permutations of $$n$$ with exactly $$k$$ cycles. The number $$c(n,k)$$ is also the coefficient of $$x^k$$ in the product $$x(x + 1)(x+2) \cdots (x+n-1)$$.

• ### Why is the hat-check problem n!/e?

The hat-check problem asks: what proportion of all permutations of $$n$$-elements have no fixed points?

• ### An example of the geometry of a combinatorial geometry

About a month ago, Dr. Federico Ardila dropped a survey paper entitled The Geometry of Geometries, surveying new and old aspects of matroid theory. I have been on a kick lately of really enjoying content he has written and also got the opportunity to chat with fellow graduate student Anastasia Nathanson about part of the paper. Dr. Galen Dorpalen-Barry taught me the value of a good example while she was a graduate student, so here is a little write up of the example which Anastasia and I came up with. Hopefully this is as helpful to you as it was to me in understanding section 4.1 of Ardila’s paper.

• ### Semimodularity and matroids

In preparation for my oral exam, I asked my advisor to quiz me on some aspects of partially ordered sets. We had a nice discussion about intervals of Young’s lattice, and distributive lattices. He then asked me about semimodular lattices, and I blanked. He then asked me what how to characterize the lattice of flats of a matroid.

• ### A semester reflection

This semester I have been TA’ing for Calculus II (MATH 1272) at the University of Minnesota. Today is the day of their final exam, so I want to take a moment to reflect on the semester. I certainly have learned a lot about teaching, but right now I’m reminded of a blog post by Terry Tao, enouraging us to “learn and relearn your field”.

• ### Combinatorics from topology

On Monday the 13th, I went to the UMN Topology seminar where I saw a talk from Dr. Mike Hill of UCLA. Admittedly I’m completely lost when it comes to most homotopy theory, but I saw “combinatorics” in the abstract and so I went to the talk hoping that I could get something out of it. A fellow UMN grad student Sasha Pevzner introduced me to the “three things” exercise for getting something out of a talk. So here are my three things from this talk: