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

Terms

a(0) =9a(1) =51a(2) =63a(3) =117a(4) =123a(5) =157a(6) =183a(7) =213a(8) =217a(9) =219a(10) =257a(11) =263a(12) =321a(13) =327a(14) =333a(15) =423a(16) =441a(17) =473a(18) =541a(19) =597a(20) =621a(21) =687a(22) =693a(23) =723a(24) =777a(25) =879a(26) =909a(27) =987a(28) =1101a(29) =1143