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Cooperative project reports new top prime number

Members of a shared computing effort says they've found the largest known prime number. It has 6.3 million digits and is the 41st entry in the Mersenne subcategory of prime numbers.

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Stephen Shankland
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A group that has harnessed the collective power of thousands of computers says it has found the largest known prime number, breaking a record it set just half a year earlier.

A prime number is evenly divisible only by 1 and itself. A Mersenne prime number, the target of the Great Internet Mersenne Prime Search (GIMPS) has more arcane requirements. The group relies on volunteers running a program in the background to search for them.

One member of the effort appeared to have found the newest record-holder on May 15. GIMPS organizer George Woltman said in an e-mail interview Tuesday that the result is being verified with separate testing over the next two to four weeks and, if confirmed, would be the largest known prime and the 41st Mersenne prime.

In November, a GIMPS member found the previous record holder, a prime number with 6.3 million digits. At that time, GIMPS was powered by 211,000 computers run by 60,000 volunteers.

The new find came more quickly than the last, which took two years of searching. "The announcement of the 40th Mersenne prime led to many new users, which increased our horsepower significantly. Also, the gap between the 41st and 40th was smaller than usual," Woltman said.

Although smaller prime numbers have applications in encryption, the largest are chiefly of academic interest. But the Electronic Frontier Foundation is sweetening the pot with a $100,000 prize for a cooperative computing project that finds a prime number with more than 10 million digits.

The new prime isn't long enough to win the award, however, Woltman said. "The EFF prize is still up for grabs," he said.

Mersenne primes are a particular variety named after Marin Mersenne, a French monk born in 1588 who investigated a particular type of prime number: 2 to the power of "p" minus one, in which "p" is an ordinary prime number.