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%I #11 Nov 20 2013 13:34:04
%S 1,1,2,2,1,2,4,4,1,2,1,2,1,2,4,2,3,4,1,2,2,1,5,4,1,2,2,4,1,6,18,20,2,
%T 4,2,3,1,4,2,2,3,6,1,12,2,1,1,96,2,4,4,2,2,1,3,3,4,6,6,4,3,6,1,4,1,2,
%U 2,1,56,2,3,8,4,4,3,4,2,4,4,3,4,4,18,20,2,8,2,2
%N Least k such that prime(n) + 2^(k+L) - 2^L is a prime, where L is the length of binary representation of prime(n): L = A070939(A000040(n)). a(n) = -1 if no such k exists.
%C Least number of 1's that must be prepended to the binary representation of prime(n) such that the result is another prime.
%C Prime(n) is in A065047 if and only if a(n) = 1.
%e a(6) = 1 because 13 in binary is 1101, and 29 (11101 in binary) is a prime.
%e a(7) = 2 because 17 in binary is 10001, and 113 (1110001 in binary) is a prime.
%e a(8) = 4 because 19 in binary is 10011, and 499 (111110011 in binary) is a prime.
%Y Cf. A000040, A070939, A065047, A094076, A023758.
%K nonn,base,less
%O 2,3
%A _Alex Ratushnyak_, Nov 17 2013