a(n) = nextprime(2^n) - 2^n.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =1a(5) =5a(6) =3a(7) =3a(8) =1a(9) =9a(10) =7a(11) =5a(12) =3a(13) =17a(14) =27a(15) =3a(16) =1a(17) =29a(18) =3a(19) =21a(20) =7a(21) =17a(22) =15a(23) =9a(24) =43a(25) =35a(26) =15a(27) =29a(28) =3a(29) =11