%I #6 Mar 30 2012 17:33:55
%S 15,4095,1152921504606846975
%N 2^(2p - 1)/2-1, where p is a Mersenne prime A000668(n).
%F a(n) = 2^(2*A000668(n)-1)/2-1 = A139296(n)-1.
%K bref,nonn
%O 1,1
%A _Omar E. Pol_, Apr 13 2008
