Mersenne prime
Number theory is famous for problems that everyone can understand and that are easy to express, but that are fiendishly difficult to prove. Here are some of our favourites. 

They've done it again! GIMPS has discovered the largest known prime number: 257,885,1611. This massive 17,425,170 digit number was discovered thanks to clever distributed computing software that uses idle computer time donated by volunteers. 
The Great Internet Mersenne Prime Search strikes again

In issue No 1 we introduced GIMPS, the Great Internet Mersenne Prime Search. Well, on Sunday 24th August 1997 they did it again. 
You may think that searching for new prime numbers is a job for supercomputers. However, on 13th November 1996, Frenchman Joel Armengaud discovered a new one using his humble PC. 