1,4

A 2-tree is recursively defined as follows: K_2 is a 2-tree and any 2-tree on n+1 vertices is obtained by joining a vertex to a 2-clique in a 2-tree on n vertices. Care is needed with the term 2-tree (and k-tree in general) because it has at least two commonly used definitions.

A036361 gives the labeled version of this sequence, which has an easy formula analagous to Cayley's formula for the number of trees.

Also, number of unlabeled 3-gonal 2-trees with n 3-gons.

F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 76, t(x), (3.5.19).

Index entries for sequences related to trees

G. Labelle, C. Lamathe and P. Leroux, Labeled and unlabeled enumeration of k-gonal 2-trees

a(1)=a(2)=a(3)=1 because: K_2, K_3 are the only 2-trees on 2 and 3 nodes and on 4 nodes, there is a also unique example obtained by joining a triangle to K_3 along an edge (thus forming K_4\e). The two graphs on 5 nodes are obtained by joining a triangle to K_4\e, either along the shared edge or along one of the non-shared edges.

Cf. A036361.

Cf. A000272 (labeled trees), A036361 (labeled 2-trees), A036362 (labeled 3-trees), A036506 (labeled 4-trees), A000055 (unlabeled trees).

Sequence in context: A050237 A050258 A051436 this_sequence A140440 A005664 A009739

Adjacent sequences: A054578 A054579 A054580 this_sequence A054582 A054583 A054584

nonn,nice

Vladeta Jovovic (vladeta(AT)eunet.rs), Apr 11 2000

Additional comments from Gordon Royle (gordon(AT)maths.uwa.edu.au), Dec 02 2002