Concatenations C1 and C2 and C3 and C4 are all prime (see the comment lines).

Terms

a(0) =16776a(1) =41719a(2) =164612a(3) =188435a(4) =188682a(5) =312184a(6) =317594a(7) =392771a(8) =397617a(9) =450413a(10) =476055a(11) =486283a(12) =492240a(13) =497913a(14) =539471a(15) =584029a(16) =620029a(17) =640883a(18) =648445a(19) =656757a(20) =903009a(21) =992790a(22) =993475a(23) =995917a(24) =1045387a(25) =1082078a(26) =1194606a(27) =1252496a(28) =1322841