Tree-optimized directed graphs

Abstract

For an additive submonoid $\mathcal{M}$ of $\mathbb{R}_{\ge 0}$, the weight of an $\mathcal{M}$-labeled directed graph is the sum of all of its edge labels, while the content is the product of the labels. Having fixed $\mathcal{M}$ and a directed tree $E$, we prove a general result on the shape of directed $\mathcal{M}$-labeled graphs $\Gamma$ of weight $N\in \mathcal{M}$ maximizing the sum of the contents of all copies $E\subset \Gamma$. This specializes to recover a result of Hajac and Tobolski on the maximal number of length-$k$ paths in a directed acyclic graph. It also applies to prove a conjecture by the same authors on the maximal sum of entries of $A^k$ for a nilpotent $\mathbb{R}_{\ge 0}$-valued square matrix $A$ whose entries add up to $N$. Finally, we apply the same techniques to obtain the maximal number of stars with $a$ arms in a directed graph with $N$ edges.