%I #4 Mar 30 2012 17:33:55
%S 4,1024,288230376151711744
%N 2^(2p - 1)/8, where p is a Mersenne prime A000668(n).
%F a(n) = 2^(2*A000668(n)-1)/8 = A139294(n)/8.
%K bref,nonn
%O 1,1
%A _Omar E. Pol_, Apr 13 2008
