## Archive for August, 2010

### Pop Quiz

Twelve questions to get the mental math joke juices flowing. Answers will be posted tomorrow.

- How many eggs can you put in an empty basket?
- How is the moon like a dollar?
- What coin doubles in value when half is taken away?
- If you can buy 8 eggs for 26 cents, how many can you buy for a penny and a quarter?
- What occurs once in a minute, twice in a week, but only once in a year?
- What goes up but never comes down?
- Why is it impossible for a human arm to be exactly 12 inches long?
- Only DEAD people can read hexadecimal. How many people can read hexadecimal?
- How do you make 7 even?
- One is the loneliest number, two’s company, and three’s a crowd. What is four and five?
- Why do statisticians hate to shop for clothes?
- The math department organizes a raffle in which the prize is announced as an infinite amount of money paid over an infinite amount of time. With the promise of such a prize, the department is able to sell lots of tickets. How could the department offer such a prize and not go broke?

### Presentation and Booksigning: Reiter’s Bookstore (DC)

I’ll be presenting (an abridged version of) my workshop Puns and Puzzles at Reiter’s Books in Washington, DC, on Saturday, September 11.

Reiter’s Books

1900 G Street NW

Washington, DC 20006

**2:00 pm**

The presentation will combine jokes from * Math Jokes 4 Mathy Folks*, some new jokes and even a comedy sketch, as well as some of my favorite mathematical puzzles. If you happen to find yourself in the nation’s capital with nothing better to do on a Saturday afternoon, please stop by to say hello.

*Math Jokes 4 Mathy Folks*will be available for sale, and after the presentation, I’ll be happy to sign a copy for you — or for the special geek in your life!

I look forward to seeing you!

### The Girl With the Dragon Tattoo

*The Girl Who Played with Fire* is the second volume in the late Stieg Larsson‘s *The Millenium Trilogy*. (Of course, you probably already knew that, since virtually everyone in North America has read this book. I mean, *someone* had to buy those 20 million copies, right?)

In this book the heroine, Lisbeth Salander, gets absorbed in recreational mathematics. She stumbles across a theorem about perfect numbers that, surprisingly, was proved by Euclid. (This is surprising because Euclid did most of his work in geometry, and a proof of his theorem about perfect numbers would rely on algebra and number theory.) The theorem appeared as Proposition IX.36 of Euclid’s *Elements.*

Stieg Larsson writes:

…a perfect number is always a multiple of two numbers, in which one number is a power of 2, and the second consists of the difference between the next power of 2 and 1. This was a refinement of Pythagoras’ equation, and [Lisbeth] could see the endless combinations:

6 = 2

^{1}(2^{2}– 1)28 = 2

^{2}(2^{3}– 1)496 = 2

^{4}(2^{5}– 1)8128 = 2

^{6}(2^{7}– 1)She could go on indefinitely without finding any numbers that would break the rule.

What Lisbeth does not state, but what is required for Euclid’s theorem to hold, is that 2^{k}(2^{k – 1} – 1) is a perfect number if and only if 2^{k – 1} is prime. She doesn’t state this — but her list of “endless combinations” only includes examples for which this is the case.

I don’t begrudge Larsson for this omission. After all, how can you be mad at the first author to sell more than one-million e-books on Amazon, especially when his most popular works were published posthumously? Besides, adding too much math to a popular fiction novel might make it a little less popular. I’m just happy that so many readers will be exposed to a little of the mathematical beauty that makes me love numbers.

Here’s a perfect quote from Descartes:

Perfect numbers, like perfect individuals, are very rare.

And a perfect joke:

Teacher: What is 14 + 14?

Student: 28.

Teacher: That’s good!

Student: Good? It’sperfect!

### Back to School

Even though parents, teachers, and kids in Alabama aren’t happy that their summer ended several weeks ago, school has started or will be starting soon for kids across the country. **Best of luck to all those returning to the classroom, regardless which side of the desk you’re on.**

Here are a few rib-ticklers about (and appropriate for) the classroom.

Father: Did you learn a lot in math class today?

Son: Apparently not! They want me to come back again tomorrow!

Why did the student eat her homework?

Because she thought it was “a piece of cake.”

A young boy asked his grandmother for help with his math homework. “I need to find the least common denominator,” he told her.

“My goodness,” his grandmother replied. “I can’t believe theystillhaven’t found that. They were looking for that when I was in school!”

### Brief Q & A

Even Georg Cantor would have trouble counting the number of mathematician light bulb jokes…

Q: How many mathematicians does it take to change a light bulb?

A: Just one. She gives it to three physicists, thus reducing it to a problem that has already been solved.

And it’s rumored that the following joke is what caused Gottfried Leibniz to lose favor with George I…

Q: What happened in the binary race?

A: Zero won.

### Random Thoughts

The following was sent to me by my friend Pat Flynn, and it may enter my email signature soon.

The derivative of my enthusiasm for mathematics is positive for all values of the independent variable.

And here are some one-liners that don’t warrant their own posts, but they’re just too good not to share…

Heisenberg might have slept here.

Old mathematicians never die; they just lose some of their functions.

Whenever four mathematicians get together, you’ll likely find a fifth.

“Take a positive integer

n. No, wait,nis too large; take a positive integerk.”

### National Tell a Joke Day

There is no shortage of fabricated holidays. I eat popcorn on National Popcorn Day (January 19), I don’t tell lies on National Honesty Day (April 30), and I break mirrors and walk under ladders with reckless abandon on Defy Superstition Day (September 13).

But there is no fabricated holiday around which I am more able to rally than National Tell a Joke Day, celebrated annually on August 16! That’s today, so happy holiday to you.

Here’s an oldie but goodie that’s just right for a day so special:

Some engineers needed to measure the height of a flag pole. They only had a measuring tape, and were getting quite frustrated trying to slide the tape up the pole. Eventually, a mathematician happens by, listens to their problem, and says he can help. He removes the pole from the ground and measures it easily. When he leaves, one of the engineers says, “Leave it to a mathematician! We need to find the height, and he gives us the length!”