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A matroid associated with a phylogenetic tree

A (pseudo-)metric $D$ on a finite set $X$ is said to be a `tree metric' if there is a finite tree with leaf set $X$ and non-negative edge weights so that, for all $x,y \in X$, $D(x,y)$ is the path distance in the tree between $x$ and $y$. It is well known that not every metric is a tree metric. However, when some such tree exists, one can always find one whose interior edges have strictly positive edge weights and that has no vertices of degree 2, any such tree is -- up to canonical isomorphism -- uniquely determined by $D$, and one does not even need all of the distances in order to fully (re-)construct the tree's edge weights in this case. Thus, it seems of some interest to investigate which subsets of $\binom{X}{2}$ suffice to determine (`lasso') these edge weights. In this paper, we use the results of a previous paper to discuss the structure of a matroid that can be associated with an (unweighted) $X-$tree $T$ defined by the requirement that its bases are exactly the `tight edge-weight lassos' for $T$, i.e, the minimal subsets $\cl$ of $\ch$ that lasso the edge weights of $T$.