My son is taking a calculus course in high school this year. While talking to him about his homework, I have come to realize that his knowledge of trigonometry is pretty weak. He said that they were supposed learn trigonometry last spring after the COVID-19 pandemic hit and school switched to fully online. Needless to…

# Author: Dave Richeson

## Puzzle: Let’s (Un)Do the Twist!

Today, while walking our dog, I had an idea for the puzzle shown below. Here is a printable pdf. I hope you enjoy it. If you would like a hint as to how to solve the puzzle, read about this puzzle from the great puzzle master Sam Loyd; it was the inspiration for my puzzle.

## Mathematicians’ Phone Passcodes

A character in a novel I was reading used the passcode 1729 for his house’s security system. He did so because of the famous Hardy-Ramanujan anecdote about the number. That got me to thinking. What would mathematicians of the past have used for their passcodes? I tweeted some ideas and got some great responses with…

## Make a Real Projective Plane (Boy’s Surface) out of Paper

I am teaching an undergraduate course in topology. We are now looking at what we get if we take a square and glue the sides together. (These are called identification spaces.) We are assuming that our spaces are made out of very stretchy rubber. So, if the space begins as a square, we could, for instance,…

## The Magnificent Möbius Band

As I write this blog post, we are all either struggling with the impact of the COVID-19 virus or waiting nervously as cases start to rise in our area. I am currently teaching remotely. My college students are scattered around the globe, and we are interacting through various online methods. This semester I am teaching…

## How to Present a Mathematical Proof or Problem

There are many useful websites containing advice on how to give a good mathematics presentation (such as those listed here). But these are written for scholars who are giving lectures on their research. They focus on organizing the talk, putting the research in context, deciding what to include or not include, designing slides, pacing and…

## Homemade Klein Bottle

One of my favorite items in my collection of mathematical objects is a glass Klein bottle made by Cliff Stoll. So I was excited when—a few years ago—I saw that it was possible to make a glass-looking Klein bottle out of a Method soap bottle. Eventually, we bought some of this soap and then used…

## Tales of Impossibility: Now Published!

I’m very excited to announce that my new book, Tales of Impossibility: The 2000-Year Quest to Solve the Mathematical Problems of Antiquity (Princeton University Press, 2019), is now available! (OK. It was published about a month ago, but I am just now getting around to blogging about it.) Like my previous book, Euler’s Gem (Princeton University…

## BraidTiles—A Mathematical Braid Puzzle

We can view braids mathematically as n strings hanging from a horizontal bar. Each piece of string runs downward and can cross neighboring strings. In the 1920s Emil Artin observed that braids of n strings form an algebraic group. To “multiply” two braids, we append the bottom of one braid with the top of another braid. The identity element in this group…

## A Numerical Crossword Puzzle

I recently made my first crossword puzzle. It was great fun. It had some mathematical clues, but it was not mathematical. So for my second crossword puzzle, I decided to make one that was 100% mathematical. Download my numerical crossword puzzle in which each cell contains a decimal point ” . ” or a digit 0…