# Recursive tree

In graph theory, a recursive tree (i.e., unordered tree) is a non-planar labeled rooted tree. A size-n recursive tree is labeled by distinct integers 1, 2, ..., n, where the labels are strictly increasing starting at the root labeled 1. Recursive trees are non-planar, which means that the children of a particular node are not ordered. E.g. the following two size-three recursive trees are the same.

       1          1
/ \   =    / \
/   \      /   \
2     3    3     2


Recursive trees also appear in literature under the name Increasing Cayley trees.

## Properties

The number of size-n recursive trees is given by

$T_{n}=(n-1)!.\,$ Hence the exponential generating function T(z) of the sequence Tn is given by

$T(z)=\sum _{n\geq 1}T_{n}{\frac {z^{n}}{n!}}=\log \left({\frac {1}{1-z}}\right).$ Combinatorically a recursive tree can be interpreted as a root followed by an unordered sequence of recursive trees. Let F denote the family of recursive trees.

$F=\circ +{\frac {1}{1!}}\cdot \circ \times F+{\frac {1}{2!}}\cdot \circ \times F*F+{\frac {1}{3!}}\cdot \circ \times F*F*F*\cdots =\circ \times \exp(F),$ where $\circ$ denotes the node labeled by 1, × the Cartesian product and $*$ the partition product for labeled objects.

By translation of the formal description one obtains the differential equation for T(z)

$T'(z)=\exp(T(z)),$ with T(0) = 0.

## Bijections

There are bijective correspondences between recursive trees of size n and permutations of size n − 1.

## Applications

Recursive trees can be generated using a simple stochastic process. Such random recursive trees are used as simple models for epidemics.