Exactly 3 digits from {1,2,3,4,5,6,7,8,9} can precede a term to form a prime.

Terms

a(0) =13a(1) =21a(2) =23a(3) =29a(4) =33a(5) =37a(6) =43a(7) =49a(8) =51a(9) =59a(10) =61a(11) =69a(12) =71a(13) =79a(14) =81a(15) =93a(16) =99a(17) =101a(18) =103a(19) =111a(20) =113a(21) =121a(22) =127a(23) =133a(24) =137a(25) =161a(26) =163a(27) =167a(28) =171a(29) =179