# Frank Morgan's Math Chat

December 17, 1998

### AMERICA'S FAVORITE NUMBER

OLD CHALLENGE. Math Chat invited each reader to send in a number. The most commonly submitted number, with the best explanation of why, wins.

ANSWER. In a tight race between 7 and p, p wins by a single vote. (p is what you get when you divide the circumference of any circle by its diameter.) Sebastian Zwicknagl reasons that all integers are equally interesting, but that p is special. Arthur Pasternak realizes that an internet audience might choose the more mathematical number. And as Dean Thomas puts it, "p is the most mathematical number I know!" Joel Foisy calls it "the best known irrational number." Rational numbers have decimal expansions that either end or repeat, such as 1/4 = .25 or 1/3 = .333..., but

p = 3.1415926535897932384626433832795028841971...

goes on forever. Amazingly enough, p comes up not only in geometry, but in almost every branch of mathematics.

Other contenders include 0, 1, 2, 17, and 70.

NEW CHALLENGE (Steve Jabloner). How many times do you think you need to role a normal die to be 90-percent sure that each of the six faces has appeared at least once? Why?