Numbers k such that k prefixed by '2' and followed by '1' is prime.

Terms

a(0) =1a(1) =4a(2) =5a(3) =7a(4) =8a(5) =11a(6) =13a(7) =14a(8) =16a(9) =22a(10) =25a(11) =28a(12) =31a(13) =34a(14) =35a(15) =37a(16) =38a(17) =41a(18) =44a(19) =52a(20) =53a(21) =55a(22) =59a(23) =62a(24) =67a(25) =71a(26) =73a(27) =74a(28) =79a(29) =80