Digit '4' concatenated with a(n) is a prime.

Terms

a(0) =1a(1) =3a(2) =7a(3) =19a(4) =21a(5) =31a(6) =33a(7) =39a(8) =43a(9) =49a(10) =57a(11) =61a(12) =63a(13) =67a(14) =79a(15) =87a(16) =91a(17) =99a(18) =111a(19) =127a(20) =129a(21) =133a(22) =139a(23) =153a(24) =157a(25) =159a(26) =177a(27) =201a(28) =211a(29) =217