Roman numerals written using 1 for I, 2 for V, 3 for X, 4 for L, 5 for C, 6 for D, 7 for M.

Terms

a(0) =1a(1) =11a(2) =111a(3) =12a(4) =2a(5) =21a(6) =211a(7) =2111a(8) =13a(9) =3a(10) =31a(11) =311a(12) =3111a(13) =312a(14) =32a(15) =321a(16) =3211a(17) =32111a(18) =313a(19) =33a(20) =331a(21) =3311a(22) =33111a(23) =3312a(24) =332a(25) =3321a(26) =33211a(27) =332111a(28) =3313a(29) =333