Prefixing, suffixing or inserting a 7 in the number anywhere gives a prime.

Terms

a(0) =1a(1) =3a(2) =9a(3) =19a(4) =27a(5) =33a(6) =39a(7) =57a(8) =87a(9) =97a(10) =159a(11) =177a(12) =187a(13) =433a(14) =487a(15) =541a(16) =603a(17) =691a(18) =717a(19) =753a(20) =757a(21) =853a(22) =949a(23) =1257a(24) =1471a(25) =1707a(26) =2277a(27) =2367a(28) =2617a(29) =2953