Smallest prime >= 4^n.

Terms

a(0) =2a(1) =5a(2) =17a(3) =67a(4) =257a(5) =1031a(6) =4099a(7) =16411a(8) =65537a(9) =262147a(10) =1048583a(11) =4194319a(12) =16777259a(13) =67108879a(14) =268435459a(15) =1073741827a(16) =4294967311a(17) =17179869209a(18) =68719476767a(19) =274877906951