Smallest natural number requiring n letters in English.

Terms

a(0) =1a(1) =4a(2) =3a(3) =11a(4) =15a(5) =13a(6) =17a(7) =24a(8) =23a(9) =73a(10) =3000a(11) =11000a(12) =15000a(13) =101a(14) =104a(15) =103a(16) =111a(17) =115a(18) =113a(19) =117a(20) =124a(21) =123a(22) =173a(23) =323a(24) =373a(25) =1104a(26) =1103a(27) =1111a(28) =1115a(29) =1113