David Richeson: Division by Zero

Parabolas and focal points II

Yesterday I wrote about parabolic mirrors. I pointed out that parallel rays hitting a parabola typically do not reflect back to a focal point. That happens only when the rays are parallel to the axis of the parabola. Then my friend Dan sent me an email encouraging me to look at the envelope of the…

Short talks at conferences

I just received an email from the MAA regarding MathFest 2009 that made me laugh. I’m scheduled to give a ten minute (10 minute!) talk in one of the sessions. Those of you who’ve given a 10 minute talk know that it is impossible to say anything in that amount of time. (This reminds me…

Parabolas and focal points

A colleague from another department stopped by my office this morning with a question about parabolas and parabolic mirrors. In the simplest terms, his question was the following: When parallel rays reflect off a parabola, do they always converge to a focal point? As we can see in the diagram below, rays that are parallel…

The Japanese Theorem

[Update: I’ve written quite a bit more about this theorem since 2009. See this page for more details.] I’ve been playing with GeoGebra for the last few days. As an exercise I decided to create applets to demonstrate the extremely beautiful Japanese Theorem. The first appearance of the Japanese theorem was as a Sangaku problem….

Pentagrams and quartic polynomials

I’m still enjoying my new-found freedom that comes with the end of the semester. I’ve gotten some research done and I’ve been able to catch up on some reading. One article that I found particularly interesting was “Quartic Polynomials and the Golden Ration,” by Harland Totland, from the June 2009 issue of Mathematics Magazine. This…

The NSF totally looks like the UAW

Here’s one for those of you who are fans of the website Totally Looks Like. The last time I was in Detroit I noticed that the emblem for the National Science Foundation totally looks like the emblem for the United Auto Workers.

Computing integer sums using l’Hôpital’s rule

Now that the busy semester is over, I’ve been able to catch up on some reading. Yesterday I read William Dunham’s article “When Euler Met l’Hôpital,” in the February 2009 issue of Mathematics Magazine. The aim of the article is to showcase some of Euler’s applications of l’Hôpital’s rule in his Institutiones calculi differentialis (1755)….

Let’s go to the carnival

The 52nd Carnival of Mathematics is underway over at The Number Warrior. I’m glad it was resurrected. Check it out! Two other mathematical tidbits. Clue in yesterday’s NY Times crossword: Calculus pioneer. The answer: EULER. Also in today’s NY Times, the headline: U.S. Jobless Rate Hits 8.9%, but Pace of Losses Eases. When I first…