http://news.yahoo.com/s/ap/20060104/ap_on_sc/largest_prime_number

By GARANCE BURKE, Associated Press Writer
Tue Jan 3, 10:09 PM ET

KANSAS CITY, Mo. - Researchers at a Missouri university have identified the largest known prime number, officials said Tuesday.

The team at Central Missouri State University, led by associate dean Steven Boone and mathematics professor Curtis Cooper, found it in mid-December after programming 700 computers years ago.

A prime number is a positive number divisible by only itself and 1 -- 2, 3, 5, 7 and so on.

The number that the team found is 9.1 million digits long. It is a Mersenne prime known as M30402457 -- that's 2 to the 30,402,457th power minus 1.

Mersenne primes are a special category expressed as 2 to the "p" power minus 1, in which "p" also is a prime number.

"We're super excited," said Boone, a chemistry professor. "We've been looking for such a number for a long time."

The discovery is affiliated with the Great Internet Mersenne Prime Search, a global contest using volunteers who run software that searches for the largest Mersenne prime.