Enumeration of Rooted Binary Unlabeled Galled Trees

Abstract

AbstractRooted binary galled trees generalize rooted binary trees to allow a restricted class of cycles, known as galls. We build upon the Wedderburn-Etherington enumeration of rooted binary unlabeled trees with n leaves to enumerate rooted binary unlabeled galled trees with n leaves, also enumerating rooted binary unlabeled galled trees with n leaves and g galls, $$0 \leqslant g \leqslant \lfloor \frac{n-1}{2} \rfloor $$ 0 ⩽ g ⩽ ⌊ n - 1 2 ⌋ . The enumerations rely on a recursive decomposition that considers subtrees descended from the nodes of a gall, adopting a restriction on galls that amounts to considering only the rooted binary normal unlabeled galled trees in our enumeration. We write an implicit expression for the generating function encoding the numbers of trees for all n. We show that the number of rooted binary unlabeled galled trees grows with $$0.0779(4.8230^n)n^{-\frac{3}{2}}$$ 0.0779 ( 4 . 8230 n ) n - 3 2 , exceeding the growth $$0.3188(2.4833^n)n^{-\frac{3}{2}}$$ 0.3188 ( 2 . 4833 n ) n - 3 2 of the number of rooted binary unlabeled trees without galls. However, the growth of the number of galled trees with only one gall has the same exponential order 2.4833 as the number with no galls, exceeding it only in the subexponential term, $$0.3910n^{\frac{1}{2}}$$ 0.3910 n 1 2 compared to $$0.3188n^{-\frac{3}{2}}$$ 0.3188 n - 3 2 . For a fixed number of leaves n, the number of galls g that produces the largest number of rooted binary unlabeled galled trees lies intermediate between the minimum of $$g=0$$ g = 0 and the maximum of $$g=\lfloor \frac{n-1}{2} \rfloor $$ g = ⌊ n - 1 2 ⌋ . We discuss implications in mathematical phylogenetics.