Number of one-sided triangulations of the disk; or flexagons of order n; or unlabeled plane trivalent trees (n-2 internal vertices, all of degree 3 and hence n leaves).

A001683

Number of one-sided triangulations of the disk; or flexagons of order n; or unlabeled plane trivalent trees (n-2 internal vertices, all of degree 3 and hence n leaves).

Terms

    a(0) =1a(1) =1a(2) =1a(3) =1a(4) =4a(5) =6a(6) =19a(7) =49a(8) =150a(9) =442a(10) =1424a(11) =4522a(12) =14924a(13) =49536a(14) =167367a(15) =570285a(16) =1965058a(17) =6823410a(18) =23884366a(19) =84155478a(20) =298377508a(21) =1063750740a(22) =3811803164a(23) =13722384546a(24) =49611801980a(25) =180072089896a(26) =655977266884

External references