Number of hex trees with n edges and having no nonroot nodes of outdegree 2.

Terms

a(0) =1a(1) =3a(2) =10a(3) =33a(4) =108a(5) =351a(6) =1134a(7) =3645a(8) =11664a(9) =37179a(10) =118098a(11) =373977a(12) =1180980a(13) =3720087a(14) =11691702a(15) =36669429a(16) =114791256a(17) =358722675a(18) =1119214746a(19) =3486784401a(20) =10847773692a(21) =33705582543a(22) =104603532030a(23) =324270949293