%N a(n) is the smallest number such that a(n)+1 is a prime and the largest power of 2 which divides it is 2^n.
%e The 4th term is 40. It is divisible by 8,40+1=41 is prime. Smaller multiples of 8 are not suitable because e.g. albeit 8|16 and 16+1=17 is a prime, but the largest power of 2 which divides 16 is not 8, it is 16. So 16 is not the 3rd, it is the 4th term here.
%Y A000040, A006093.
%A _Labos Elemer_, Nov 02 2000
%E More terms from Larry Reeves (larryr(AT)acm.org), Nov 03 2000