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

Terms

a(0) =5a(1) =17a(2) =103a(3) =127a(4) =157a(5) =191a(6) =200a(7) =201a(8) =202a(9) =203a(10) =204a(11) =205a(12) =206a(13) =207a(14) =208a(15) =209a(16) =210a(17) =227a(18) =234a(19) =235a(20) =236a(21) =237a(22) =238a(23) =258a(24) =259a(25) =260a(26) =261a(27) =262a(28) =307a(29) =347