2       2^2-1 = 3 is prime
3       2^3-1 = 7 is prime
5       2^5-1 = 31 is prime
7       2^7-1 = 127 is prime
11      2^11-1 = 2047 is not prime
13      2^13-1 = 8191 is not prime
17      2^17-1 = 131071 is not prime
19      2^19-1 = 524287 is not prime
23      2^23-1 = 8388607 is not prime
29      2^29-1 = 536870911 is not prime