a(n) = nextprime(4^n)-4^n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =1a(5) =7a(6) =3a(7) =27a(8) =1a(9) =3a(10) =7a(11) =15a(12) =43a(13) =15a(14) =3a(15) =3a(16) =15a(17) =25a(18) =31a(19) =7a(20) =15a(21) =15a(22) =7a(23) =15a(24) =21a(25) =55a(26) =21a(27) =159a(28) =81a(29) =69