2n concatenated with n.

Terms

a(0) =21a(1) =42a(2) =63a(3) =84a(4) =105a(5) =126a(6) =147a(7) =168a(8) =189a(9) =2010a(10) =2211a(11) =2412a(12) =2613a(13) =2814a(14) =3015a(15) =3216a(16) =3417a(17) =3618a(18) =3819a(19) =4020a(20) =4221a(21) =4422a(22) =4623a(23) =4824a(24) =5025a(25) =5226a(26) =5427a(27) =5628a(28) =5829a(29) =6030