a(n) is the concatenation of n and 4n.

Terms

a(0) =14a(1) =28a(2) =312a(3) =416a(4) =520a(5) =624a(6) =728a(7) =832a(8) =936a(9) =1040a(10) =1144a(11) =1248a(12) =1352a(13) =1456a(14) =1560a(15) =1664a(16) =1768a(17) =1872a(18) =1976a(19) =2080a(20) =2184a(21) =2288a(22) =2392a(23) =2496a(24) =25100a(25) =26104a(26) =27108a(27) =28112a(28) =29116a(29) =30120