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2^(2p - 1)/16, where p is a Mersenne prime A000668(n).
3

%I #4 Mar 30 2012 17:33:55

%S 2,512,144115188075855872

%N 2^(2p - 1)/16, where p is a Mersenne prime A000668(n).

%F a(n) = 2^(2*A000668(n)-1)/16 = A139294(n)/16.

%K bref,nonn

%O 1,1

%A _Omar E. Pol_, Apr 13 2008