From the graphic here you see that Mersenne primes are in the form of (2^p)-1. Pick any number for p. Use that as the exponent for 2. Then subtract 1 from the product. If the difference is prime, the prime is called a Mersenne prime.
Read more at Mersenne Primes: History, Theorems and Lists, maintained by Chris K. Caldwell.
This has all happened because of my new Dell PC. I'm still a MacAddict but have added this machine (a gift from John) to ease my photography problems. And while I'm at it, I can do other stuff, like GIMPS, that I couldn't do on a Mac.
I'm going to re-submerge myself in more caffeine and smoke now . . .