Number of letters in English name for n increases at these numbers.

Terms

a(0) =1a(1) =3a(2) =11a(3) =13a(4) =17a(5) =21a(6) =23a(7) =73a(8) =101a(9) =103a(10) =111a(11) =113a(12) =117a(13) =121a(14) =123a(15) =173a(16) =323a(17) =373a(18) =1103a(19) =1111a(20) =1113a(21) =1117a(22) =1121a(23) =1123a(24) =1173a(25) =1323a(26) =1373a(27) =3323a(28) =3373a(29) =11373