%I #4 Mar 30 2012 17:33:55
%S 1,511,144115188075855871
%N (2^(2p - 1)/16)-1, where p is a Mersenne prime A000668(n).
%F a(n) = (2^(2*A000668(n)-1)/16)-1 = (A139294(n)/16)-1.
%K bref,nonn
%O 1,2
%A _Omar E. Pol_, Apr 13 2008