Decimal concatenation of n, n+1, and n+2.

Terms

a(0) =12a(1) =123a(2) =234a(3) =345a(4) =456a(5) =567a(6) =678a(7) =789a(8) =8910a(9) =91011a(10) =101112a(11) =111213a(12) =121314a(13) =131415a(14) =141516a(15) =151617a(16) =161718a(17) =171819a(18) =181920a(19) =192021a(20) =202122a(21) =212223a(22) =222324a(23) =232425a(24) =242526a(25) =252627a(26) =262728a(27) =272829a(28) =282930a(29) =293031