Precisely defined yet enticingly elusive, prime numbers occupy a central place in number theory. Evenly divisible only by themselves and 1, these special integers–2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, and so on–pose all sorts of conundrums.
There is no known formula for primes–no combination of algebraic operations on n that will generate, with a few turns of the mathematical crank, the nth prime. To find the 99th prime, all you can do is make an ordered list of prime numbers and count to the 99th.