Start with 0; then add one to each single digit.

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =4a(5) =5a(6) =6a(7) =7a(8) =8a(9) =9a(10) =10a(11) =21a(12) =32a(13) =43a(14) =54a(15) =65a(16) =76a(17) =87a(18) =98a(19) =109a(20) =210a(21) =321a(22) =432a(23) =543a(24) =654a(25) =765a(26) =876a(27) =987a(28) =1098a(29) =2109