## Archive for March 13, 2011

### Book Review: *Prime Curios*

At the Virginia Council of Teachers of Mathematics (VCTM) conference in Richmond last Friday, I had an incredibly fun time giving an hour-long presentation titled (what else?) *Math Jokes 4 Mathy Folks*. It was a mixture of stand-up comedy, number tricks, and a brief explanation of why a laughing student is a learning student.

The joke that received the best response? I posted the following image…

…and then I said, “Holy shift! Look at the asymptote on that mother function!”

After my presentation, a gentleman approached me, turned to a page in a book, and read a joke to me:

What do you call someone who hunts for Mersenne primes only for the prize money?

A Mersennary.

He then handed me a copy of a fascinating little book, *Prime Curios: The Dictionary of Prime Number Trivia*.

Turns out, the gentleman was G. L. Honaker. He and Chris Caldwell, a professor of mathematics at University of Tennessee-Martin, are the authors of *Prime Curios*. Chris maintains The Prime Pages, a web site with 20,000 pages of prime number trivia.

The book contains quite a few nuggets worth mentioning:

- If you concatenate the positive odd integers from 1 to 97, the result is a prime number.
- The chance that no pair of 53 people in a room have the same birthday is approximately 1/53.
- The prime number 369,119 divides the sum of all prime numbers less than 369,119.

The book contains more curios like this… in fact, there are 2,148 more of them, according to the description on the back of the book. It’s a fun book, especially if you’re a big number dork like I am. Mathy folks might enjoy it, but for sure you should check out The Prime Pages.

### A Cool Quick Trick for Pi Day

To some extent, I’m anti‑Pi Day. I think it has to do with the predictability of celebrations — everyone serves pie, does circle problems, and says things like, “I’m like π: irrational, but well-rounded!”

So, I was thinking that I would boycott Pi Day this year by not posting anything about the holiday on the MJ4MF blog. Then I discovered a cool trick. It was attributed to Martin Gardner on a web site, but I can’t verify the source. I think I’ve read every book by MG, and I’ve never seen it before.

Anyway, here’s the trick.

Write all 26 letters of the alphabet, but start with the letter J:

JKLMNOPQRSTUVWXYZABCDEFGHI

Then, remove all the letters that have vertical symmetry:

JKL N PQRS Z BCDEFG

Now, count the letters that remain in each subset: 3 1 4 1 6.

When I did this trick at a K‑12 math teachers’ conference recently, I wrote the numbers under each group. But I wasn’t sure that everyone would recognize the digits. So I drew an exaggerated decimal point between the 3 and 1, and I stated, “If you don’t know why this is relevant with Pi Day just around the corner, you’ve really missed the *point*.”