David Richeson: Division by Zero

Ambigram: pie≠3.14

I thought the first comment on this article was funny. It says that pie and 3.14 are mirror images, if written in a certain way. Regular readers may recall that I had fun creating ambigrams a few months ago. This blog comment inspired me to whip up a quick ambigram exhibiting this symmetry. I like…

Last Sunday was a perfect day

Most geeky math types (like me) already know about pi day (March 14… 3/14, get it?). Writing in The Times Online, Marcus du Sautoy suggests a new math holiday: June 28. He suggests calling this day the World Math Day (actually, he suggests World Maths Day). Why? What is so mathematical about June 28? June 28 can…

Putting CVs online

I’ve had my CV online for quite some time. For a while I’d type it up, export it as a pdf, then upload it to my website. The problem was that I would only upload it once every six months or once a year—it was perpetually out of date. Then I made the switch to…

Indeterminate form in The New Yorker

In Calculus II we teach our students about a variety of indeterminate forms: , , , etc. I was reminded of another indeterminate form when reading Malcolm Gladwell’s thought-provoking (negative) review of the book Free: The Future of a Radical Price, by Chris Anderson (editor of Wired). The review appears in The New Yorker (that you can read…

The famous trick donkeys: a Sam Loyd puzzle

Several years ago a colleague and I made paper copies of this famous old puzzle to distribute to prospective mathematics students. It is a fantastic puzzle, so I thought I’d post it here again. From what I can tell, the puzzle was invented in 1871 by Sam Loyd (1842–1911), probably history’s most famous “puzzler.” It…

Carnot’s Theorem

Here’s a neat theorem from geometry. Begin with any triangle. Let R be the radius of its circumscribed circle and r be the radius of its inscribed circle. Let a, b, and c be the signed distances from the center of the circumscribed circle to the three sides. The sign of a, b, and c…

Euler’s Gem is the #2 best-selling mathematics book for libraries

It was a nice surprise to read this blog post at the Princeton University Press blog. Apparently my book (Euler’s Gem) is currently the number 2 best-selling mathematics book for libraries (according to the Library Journal). Cool! It was second to The Princeton Companion to Mathematics, which was edited by Fields Medal winner Tim Gowers. (By…

Three cool facts about rotations of the circle

I was playing around with GeoGebra and made this applet about one of the simplest, but most intersting dynamical systems: the rigid rotation of a circle. Let me tell you a little about this fascinating subject. Let denote a circle. For simplicity, let’s think of it as a circle with circumference 1. Let be any…

Keith Devlin totally looks like Graham Nash

I was watching (and loving) the PBS American Masters documentary on Neil Young last night. There were several interview snippets with Graham Nash. Every time he came on he reminded me of Keith Devlin (NPR’s “The Math Guy”). They look similar, are both British, are about the same age, and both want to teach our…