%I #2 Mar 30 2012 17:33:50
%S 7,127,131071,2147483647,618970019642690137449562111
%N Bisection of Mersenne primes A000668.
%F a(n) = A000668(2*n).
%e a(3) = 131071 because the (2*3=6)th Mersenne prime is 131071.
%Y Cf. A000668, A134721.
%K easy,nonn
%O 1,1
%A _Omar E. Pol_, Nov 11 2007