Concatenations C1 and C2 and C3 are all prime (see the comment lines).

Terms

a(0) =63a(1) =333a(2) =621a(3) =3339a(4) =4557a(5) =8127a(6) =13977a(7) =16827a(8) =18177a(9) =21921a(10) =27393a(11) =34989a(12) =40023a(13) =40047a(14) =46917a(15) =59727a(16) =61203a(17) =79467a(18) =94767a(19) =95079a(20) =100821a(21) =102771a(22) =104859a(23) =106527a(24) =113883a(25) =114123a(26) =143433a(27) =147123a(28) =156201a(29) =158409