a(n) = number of letters in a(n-1), a(1) = 1 (in English).

Terms

a(0) =1a(1) =3a(2) =5a(3) =4a(4) =4a(5) =4a(6) =4a(7) =4a(8) =4a(9) =4a(10) =4a(11) =4a(12) =4a(13) =4a(14) =4a(15) =4a(16) =4a(17) =4a(18) =4a(19) =4a(20) =4a(21) =4a(22) =4a(23) =4a(24) =4a(25) =4a(26) =4a(27) =4a(28) =4a(29) =4