The largest known prime number now has 17 million digits.

Prime numbers are those which are indivisible by any number other than itself and one.

There is an infinite number of these numbers, but there is no easy formula for finding them - meaning that mathematicians still search for new additions to the list.

To do that they need powerful computers which use brute force to check numbers until one comes up a winner.

One of the projects searching for new primes is the Great Internet Mersenne Prime Search (GIMPS) which is a distributed network of computers looking for a certain rare type of prime which follows the form 2*P - 1, where P is a prime number.

Curtis Cooper at the University of Central Missouri in Warrensburg found the new number using GIMPS.

The number - 2 to the power of 57,885,161 - 1, is four million digits longer than the previous record holder, which was found in 2008.

All of the top 10 prime numbers were found by GIMPS, and are Mersenne numbers.

One of Cooper's 1,000 computers hooked up to GIMPS spent 39 days proving the number was prime.

The Electronic Frontier Foundatio has offered $150,000 and $250,000 to anyone who can find a prime number with 100 million and a billion digits respectively.

Cooper will receive just $3,000 from GIMPS for his contribution.