×

The vertical profile of embedded trees. (English) Zbl 1253.05081

Summary: Consider a rooted binary tree with \(n\) nodes. Assign with the root the abscissa 0, and with the left (resp. right) child of a node of abscissa \(i\) the abscissa \(i-1\) (resp. \(i+1\)). We prove that the number of binary trees of size \(n\) having exactly \(n_i\) nodes at abscissa \(i\), for \(l \leq i \leq r\) (with \(n = \sum_i n_i)\), is \[ \frac{n_0}{n_l n_r} {{n_{-1}+n_1} \choose {n_0-1}} \prod_{l\leq i\leq r \atop i\not = 0}{{n_{i-1}+n_{i+1}-1} \choose {n_i-1}}, \] with \(n_{l-1}=n_{r+1}=0\). The sequence \((n_l, \dots, n_{-1};n_0, \dots n_r)\) is called the vertical profile of the tree. The vertical profile of a uniform random tree of size \(n\) is known to converge, in a certain sense and after normalization, to a random mesure called the integrated superbrownian excursion, which motivates our interest in the profile. We prove similar looking formulas for other families of trees whose nodes are embedded in \(Z\). We also refine these formulas by taking into account the number of nodes at abscissa \(j\) whose parent lies at abscissa \(i\), and/or the number of vertices at abscissa \(i\) having a prescribed number of children at abscissa \(j\), for all \(i\) and \(j\). Our proofs are bijective.

MSC:

05C30 Enumeration in graph theory
05A15 Exact enumeration problems, generating functions
05C05 Trees
05C80 Random graphs (graph-theoretic aspects)