The roster of prime numbers—those numbers divisible only by 1 and themselves—has a new top dog. On Feb. 18, the computer-based Great Internet Mersenne Prime Search (GIMPS) turned up the largest known prime number, whose formula is 2 to the 25,964,951st power minus 1. The new prime is a whopping 7,816,230 digits long, making it more than half-a-million digits longer than the previous record-holder. The number would completely fill 58 issues of Science News.
The new champion is a so-called Mersenne prime, named after the 17th-century monk Marin Mersenne who formulated a famous but incorrect conjecture about these numbers. Mersenne numbers have the form 2p – 1, where p is a prime. Written in base 2, such a number consists simply of the digit 1 repeated p times. For example, the Mersenne numbers in base 2 for p = 2 and p = 3 are 11 and 111, respectively.