15. Last December 7, a computer owned by Patrick Laroche from Florida, USA determined that the number $2^{82,589,933}-1$ is a prime number. This number had a whopping $24,862,048$ digits, and is currently the largest known prime number. The computer used software provided by the GIMPS, which is a distributed computing project on the internet trying to find prime numbers of the form $2^{p}-1$. What do you call such prime numbers?
Answer: Mersenne primes