146 research outputs found
Blocks and Cut Vertices of the Buneman Graph
Given a set \Sg of bipartitions of some finite set of cardinality at
least 2, one can associate to \Sg a canonical -labeled graph \B(\Sg),
called the Buneman graph. This graph has several interesting mathematical
properties - for example, it is a median network and therefore an isometric
subgraph of a hypercube. It is commonly used as a tool in studies of DNA
sequences gathered from populations. In this paper, we present some results
concerning the {\em cut vertices} of \B(\Sg), i.e., vertices whose removal
disconnect the graph, as well as its {\em blocks} or 2-{\em connected
components} - results that yield, in particular, an intriguing generalization
of the well-known fact that \B(\Sg) is a tree if and only if any two splits
in \Sg are compatible
Computing the blocks of a quasi-median graph
Quasi-median graphs are a tool commonly used by evolutionary biologists to
visualise the evolution of molecular sequences. As with any graph, a
quasi-median graph can contain cut vertices, that is, vertices whose removal
disconnect the graph. These vertices induce a decomposition of the graph into
blocks, that is, maximal subgraphs which do not contain any cut vertices. Here
we show that the special structure of quasi-median graphs can be used to
compute their blocks without having to compute the whole graph. In particular
we present an algorithm that, for a collection of aligned sequences of
length , can compute the blocks of the associated quasi-median graph
together with the information required to correctly connect these blocks
together in run time , independent of the size of the
sequence alphabet. Our primary motivation for presenting this algorithm is the
fact that the quasi-median graph associated to a sequence alignment must
contain all most parsimonious trees for the alignment, and therefore
precomputing the blocks of the graph has the potential to help speed up any
method for computing such trees.Comment: 17 pages, 2 figure
The polytopal structure of the tight-span of a totally split-decomposable metric
The tight-span of a finite metric space is a polytopal complex that has appeared in several areas of mathematics. In this paper we determine the polytopal structure of the tight-span of a totally split-decomposable (finite) metric. These metrics are a generalization of tree-metrics and have importance within phylogenetics. In previous work, we showed that the cells of the tight-span of such a metric are zonotopes that are polytope isomorphic to either hypercubes or rhombic dodecahedra. Here, we extend these results and show that the tight-span of a totally split-decomposable metric can be broken up into a canonical collection of polytopal complexes whose polytopal structures can be directly determined from the metric. This allows us to also completely determine the polytopal structure of the tight-span of a totally split-decomposable metric. We anticipate that our improved understanding of this structure may lead to improved techniques for phylogenetic inference
Fundamental polytopes of metric trees via parallel connections of matroids
We tackle the problem of a combinatorial classification of finite metric
spaces via their fundamental polytopes, as suggested by Vershik in 2010. In
this paper we consider a hyperplane arrangement associated to every split
pseudometric and, for tree-like metrics, we study the combinatorics of its
underlying matroid. We give explicit formulas for the face numbers of
fundamental polytopes and Lipschitz polytopes of all tree-like metrics, and we
characterize the metric trees for which the fundamental polytope is simplicial.Comment: 20 pages, 2 Figures, 1 Table. Exposition improved, references and new
results (last section) adde
Uprooted Phylogenetic Networks
The need for structures capable of accommodating complex evolutionary signals such as those found in, for example, wheat has fueled research into phylogenetic networks. Such structures generalize the standard model of a phylogenetic tree by also allowing for cycles and have been introduced in rooted and unrooted form. In contrast to phylogenetic trees or their unrooted versions, rooted phylogenetic networks are notoriously difficult to understand. To help alleviate this, recent work on them has also centered on their “uprooted” versions. By focusing on such graphs and the combinatorial concept of a split system which underpins an unrooted phylogenetic network, we show that not only can a so-called (uprooted) 1-nested network N be obtained from the Buneman graph (sometimes also called a median network) associated with the split system Σ(N)Σ(N) induced on the set of leaves of N but also that that graph is, in a well-defined sense, optimal. Along the way, we establish the 1-nested analogue of the fundamental “splits equivalence theorem” for phylogenetic trees and characterize maximal circular split systems
Injective split systems
A split system on a finite set , , is a set of
bipartitions or splits of which contains all splits of the form
, . To any such split system we can
associate the Buneman graph which is essentially a
median graph with leaf-set that displays the splits in . In
this paper, we consider properties of injective split systems, that is, split
systems with the property that for any 3-subsets
in , where denotes the median in
of the three elements in considered as leaves in
. In particular, we show that for any set there
always exists an injective split system on , and we also give a
characterization for when a split system is injective. We also consider how
complex the Buneman graph needs to become in order for
a split system on to be injective. We do this by introducing a
quantity for which we call the injective dimension for , as well as
two related quantities, called the injective 2-split and the rooted-injective
dimension. We derive some upper and lower bounds for all three of these
dimensions and also prove that some of these bounds are tight. An underlying
motivation for studying injective split systems is that they can be used to
obtain a natural generalization of symbolic tree maps. An important consequence
of our results is that any three-way symbolic map on can be represented
using Buneman graphs.Comment: 22 pages, 3 figure
The leafage of a chordal graph
The leafage l(G) of a chordal graph G is the minimum number of leaves of a
tree in which G has an intersection representation by subtrees. We obtain upper
and lower bounds on l(G) and compute it on special classes. The maximum of l(G)
on n-vertex graphs is n - lg n - (1/2) lg lg n + O(1). The proper leafage l*(G)
is the minimum number of leaves when no subtree may contain another; we obtain
upper and lower bounds on l*(G). Leafage equals proper leafage on claw-free
chordal graphs. We use asteroidal sets and structural properties of chordal
graphs.Comment: 19 pages, 3 figure
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