Prefixing, suffixing or inserting a 9 in the number anywhere gives a prime.

Terms

a(0) =7a(1) =19a(2) =37a(3) =41a(4) =91a(5) =199a(6) =209a(7) =239a(8) =311a(9) =539a(10) =587a(11) =661a(12) =749a(13) =923a(14) =931a(15) =941a(16) =967a(17) =1009a(18) =1079a(19) =1139a(20) =1997a(21) =2717a(22) =2959a(23) =3971a(24) =3979a(25) =4559a(26) =4993a(27) =4999a(28) =5393a(29) =5629