by Joe Palca
Morning Edition, June 16, 2009 · A computer professional in Norway, with the help of an online computing project, has discovered a new Mersenne prime. This sought-after number represents the 47th Mersenne prime discovered since ancient Greek mathematicians first uncovered them. These primes are called the "jewels" of number theory, and it takes a huge computing system about two or three weeks to test a single number to see if it could lead to a Mersenne prime.
For those of you for whom basic math is a distant memory, a reminder: Primes are numbers that are divisible by only the number 1 and themselves. So 2 is prime; so are 3, 5, 7 and so on. The year 2003 was a prime year, and 2011 will be as well.
Not Just Any Prime
Mersenne primes are a special class of prime, and they have a particular formula. The formula for finding Mersenne primes. The number n is a prime, and the result is prime. And what makes the Mersenne primes so interesting is how rare they are. And their gargantuan size. Ancient Greek mathematicians were the first to describe Mersenne primes, and, up until now, only 46 had been discovered. This most recent one — though not the largest — is a whopper at nearly 13 million digits long.
It was found as part of the Great Internet Mersenne Prime Search, GIMPS. GIMPS involves tens of thousand of computers churning away, searching for new Mersenne primes. In order to test potential primes, "not only do you have to multiply a 13 million digit number by a 13 million digit number, but you have to do that about 13 million times," says mathematician Chris Caldwell of the University of Tennessee. "And that just takes a tremendous amount of computation."
When I first reported on the Great Internet Mersenne Prime Search last April 10, I exhorted NPR listeners to download the search software and join the effort. The new Mersenne was discovered by someone who was already a prime hunter. Odd Magnar Strindmo from Melhus, Norway, has been part of GIMPS since it began in 1996. Now, finding a new Mersenne prime is exciting all by itself, but it is especially exciting because of a bet I made with George Woltman, who runs the Great Internet Mersenne Prime Search. I bet the next one would be found before 2012, helped along by all those NPR listeners who I hoped would download the program.
Woltman emailed me saying he felt a little silly losing the bet so soon. But I have a confession to make. I meant the next largest one; the 47th Mersenne prime is actually 141,125 digits smaller than one that was discovered last year. So I won on a technicality, and I'm willing to carry on the bet until the largest one is found. After all, we didn't bet any money, so I can afford to be magnanimous.
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