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

Terms

a(0) =1a(1) =5a(2) =10a(3) =50a(4) =100a(5) =500a(6) =1000a(7) =2a(8) =4a(9) =6a(10) =9a(11) =11a(12) =15a(13) =20a(14) =40a(15) =51a(16) =55a(17) =60a(18) =90a(19) =101a(20) =105a(21) =110a(22) =150a(23) =200a(24) =400a(25) =501a(26) =505a(27) =510a(28) =550a(29) =600