a(n) is the concatenation of n and 3n.

Terms

a(0) =13a(1) =26a(2) =39a(3) =412a(4) =515a(5) =618a(6) =721a(7) =824a(8) =927a(9) =1030a(10) =1133a(11) =1236a(12) =1339a(13) =1442a(14) =1545a(15) =1648a(16) =1751a(17) =1854a(18) =1957a(19) =2060a(20) =2163a(21) =2266a(22) =2369a(23) =2472a(24) =2575a(25) =2678a(26) =2781a(27) =2884a(28) =2987a(29) =3090