a(n) is the concatenation of n and 2n.

Terms

a(0) =12a(1) =24a(2) =36a(3) =48a(4) =510a(5) =612a(6) =714a(7) =816a(8) =918a(9) =1020a(10) =1122a(11) =1224a(12) =1326a(13) =1428a(14) =1530a(15) =1632a(16) =1734a(17) =1836a(18) =1938a(19) =2040a(20) =2142a(21) =2244a(22) =2346a(23) =2448a(24) =2550a(25) =2652a(26) =2754a(27) =2856a(28) =2958a(29) =3060