a(n) is the least prime such that the multiplicative order of 4 mod a(n) equals n.

Terms

a(0) =3a(1) =5a(2) =7a(3) =17a(4) =11a(5) =13a(6) =43a(7) =257a(8) =19a(9) =41a(10) =23a(11) =241a(12) =2731a(13) =29a(14) =151a(15) =65537a(16) =43691a(17) =37a(18) =174763a(19) =61681a(20) =337a(21) =397a(22) =47a(23) =97a(24) =251a(25) =53a(26) =87211a(27) =15790321a(28) =59a(29) =61