Entertaining the idea of a peculiar long number such as pi puts me in mind of its cousins twice removed, prime numbers, which have so many strange properties. Prime numbers are those numbers, such as 3, 5, 7, 11, and so on, that can be cleanly divided only by one and themselves. Primes and pi suggest a benign infinity, a pleasing order—pi because of its endlessness and its relation to the circle, and primes because no matter how far you travel on the number line you will always encounter a prime, as Euclid proved in 300 B.C. Recently, Yitang Zhang solved a problem involving prime numbers called bounded gaps that had been open for more than a hundred years. Zhang proved that no matter how far you go on the number line, even to the range where the numbers would fill many books, there will, on an infinite number of occasions, be two prime numbers within seventy million places of each other. (Other mathematicians have reduced this gap to two hundred and forty six.) Before, it had not been known whether any such range applied to prime numbers, which seem to behave, especially as they get larger, as if they appear at random.