Number of equivalence classes of ballot paths of length n for the string ddd.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =2a(7) =3a(8) =5a(9) =7a(10) =10a(11) =13a(12) =20a(13) =28a(14) =45a(15) =65a(16) =101a(17) =143a(18) =222a(19) =317a(20) =500a(21) =726a(22) =1143a(23) =1661a(24) =2608a(25) =3796a(26) =5983a(27) =8764a(28) =13835a(29) =20335