%I
%S 1,5,9,29,61,119,249,507,1017,2045,4047,8181,16381,32765,65521,131069,
%T 262137,524277,1048567,2097145,4194285,8388597,16777185,33554421,
%U 67108821,134217725,268435405,536870909,1073741811,2147483619,4294967275,8589934569,17179869177
%N Largest k such that (2^nk)*2^nk is a prime number.
%e a(2)=1 because (2^21)*2^21 = 11 is prime,
%e a(3)=5 because (2^35)*2^35 = 19 is prime,
%e a(4)=7 because (2^47)*2^47 = 137 is prime.
%o (PARI) a(n)=forstep(k=2^n1,1,1, ispseudoprime((2^nk)*2^nk) & return(k)) \\ _M. F. Hasler_, Jun 17 2011
%Y Cf. A000040.
%K nonn
%O 2,2
%A _JuriStepan Gerasimov_, Jun 16 2011
