Number of digits in n equals number of syllables in English name of n.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =5a(5) =6a(6) =8a(7) =9a(8) =13a(9) =14a(10) =15a(11) =16a(12) =18a(13) =19a(14) =20a(15) =30a(16) =40a(17) =50a(18) =60a(19) =80a(20) =90a(21) =100a(22) =200a(23) =300a(24) =400a(25) =500a(26) =600a(27) =800a(28) =900a(29) =7000