Numbers where k-th digit from right is either 0 or k.

Terms

a(0) =0a(1) =1a(2) =20a(3) =21a(4) =300a(5) =301a(6) =320a(7) =321a(8) =4000a(9) =4001a(10) =4020a(11) =4021a(12) =4300a(13) =4301a(14) =4320a(15) =4321a(16) =50000a(17) =50001a(18) =50020a(19) =50021a(20) =50300a(21) =50301a(22) =50320a(23) =50321a(24) =54000a(25) =54001a(26) =54020a(27) =54021a(28) =54300a(29) =54301