Numbers k such that k prefixed by '9' and followed by '1' is prime.

Terms

a(0) =1a(1) =4a(2) =7a(3) =9a(4) =15a(5) =16a(6) =18a(7) =22a(8) =24a(9) =28a(10) =31a(11) =34a(12) =37a(13) =39a(14) =42a(15) =43a(16) =46a(17) =49a(18) =51a(19) =52a(20) =55a(21) =60a(22) =63a(23) =66a(24) =72a(25) =78a(26) =79a(27) =81a(28) =85a(29) =87