a(n) = 2^prime(n) + prime(n).

Terms

a(0) =6a(1) =11a(2) =37a(3) =135a(4) =2059a(5) =8205a(6) =131089a(7) =524307a(8) =8388631a(9) =536870941a(10) =2147483679a(11) =137438953509