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

Terms

a(0) =33a(1) =51a(2) =53a(3) =63a(4) =111a(5) =123a(6) =129a(7) =211a(8) =237a(9) =273a(10) =357a(11) =489a(12) =519a(13) =573a(14) =597a(15) =609a(16) =639a(17) =651a(18) =653a(19) =657a(20) =669a(21) =681a(22) =687a(23) =747a(24) =753a(25) =819a(26) =831a(27) =873a(28) =891a(29) =987