Roman numerals with 1 letter, in alphabetical order; then those with 2 letters, etc.

Terms

a(0) =100a(1) =500a(2) =1a(3) =50a(4) =1000a(5) =5a(6) =10a(7) =200a(8) =400a(9) =101a(10) =150a(11) =900a(12) =105a(13) =110a(14) =600a(15) =501a(16) =550a(17) =505a(18) =510a(19) =2a(20) =4a(21) =9a(22) =51a(23) =55a(24) =60a(25) =1100a(26) =1500a(27) =1001a(28) =1050a(29) =2000