a(n) = prime(4^n).

Terms

a(0) =2a(1) =7a(2) =53a(3) =311a(4) =1619a(5) =8161a(6) =38873a(7) =180503a(8) =821641a(9) =3681131a(10) =16290047a(11) =71378569a(12) =310248241a(13) =1339484197a(14) =5750079047a(15) =24563311309a(16) =104484802057a(17) =442795487221