Horton Law in Self-Similar Trees

TL;DR

This paper investigates the mathematical properties of self-similar trees, establishing conditions under which the Horton law holds, thereby providing a rigorous foundation for a law observed in natural branching systems.

Contribution

It introduces a formal framework for self-similar trees, linking Tokunaga coefficients to the Horton law and establishing necessary and sufficient conditions for its validity.

Findings

Horton law holds under specific self-similarity conditions.

Invariance of Tokunaga coefficients implies the Horton law.

Necessary and sufficient condition involves the growth rate of Tokunaga coefficients.

Abstract

Self-similarity of random trees is related to the operation of pruning. Pruning $R$ cuts the leaves and their parental edges and removes the resulting chains of degree-two nodes from a finite tree. A Horton-Strahler order of a vertex $v$ and its parental edge is defined as the minimal number of prunings necessary to eliminate the subtree rooted at $v$. A branch is a group of neighboring vertices and edges of the same order. The Horton numbers $N_k[K]$ and $N_{ij}[K]$ are defined as the expected number of branches of order $k$, and the expected number of order-$i$ branches that merged order-$j$ branches, $j>i$, respectively, in a finite tree of order $K$. The Tokunaga coefficients are defined as $T_{ij}[K]=N_{ij}[K]/N_j[K]$. The pruning decreases the orders of tree vertices by unity. A rooted full binary tree is said to be mean-self-similar if its Tokunaga coefficients are invariant with respect to pruning: $T_k:=T_{i,i+k}[K]$. We show that for self-similar trees, the condition $\limsup(T_k)^{1/k}<\infty$ is necessary and sufficient for the existence of the strong Horton law: $N_k[K]/N_1[K] \rightarrow R^{1-k}$, as $K \rightarrow \infty$ for some $R>0$ and every $k\geq 1$. This work is a step toward providing rigorous foundations for the Horton law that, being omnipresent in natural branching systems, has escaped so far a formal explanation.