Use Dr.Rackt:

If both n and 2ⁿ 1 are primes, then n is said to be a Mersenne prime. The first few Mersenne primes are 2, 3, 5, 7, 13, 17, 19, 31, and 61. Use the primality routines in SICP 1.2.6 to write a procedure for finding such primes (or use the built in prime?). It should take two parameters the starting n and a predicate to test for primality, and return the next Mersenne prime. (If you find a ten million digit prime number, you can pick up a $100,000 prize and forget all about Lisp; see http://www. mersenne.org/.) An example of calling this function might look like this:

 (next-mersenne 20 prime?) 

which should return 31.