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

Terms

a(0) =657a(1) =4929a(2) =10647a(3) =18291a(4) =20907a(5) =22653a(6) =27381a(7) =28053a(8) =28671a(9) =42471a(10) =48441a(11) =57363a(12) =69651a(13) =79569a(14) =89853a(15) =93261a(16) =95967a(17) =101283a(18) =110763a(19) =119481a(20) =128613a(21) =145719a(22) =149409a(23) =174993a(24) =201417a(25) =204861a(26) =205329a(27) =221883a(28) =248391a(29) =269199