28 research outputs found
Geometric Reasoning with polymake
The mathematical software system polymake provides a wide range of functions
for convex polytopes, simplicial complexes, and other objects. A large part of
this paper is dedicated to a tutorial which exemplifies the usage. Later
sections include a survey of research results obtained with the help of
polymake so far and a short description of the technical background
Computing the bounded subcomplex of an unbounded polyhedron
We study efficient combinatorial algorithms to produce the Hasse diagram of
the poset of bounded faces of an unbounded polyhedron, given vertex-facet
incidences. We also discuss the special case of simple polyhedra and present
computational results.Comment: 16 page
The decomposition of the hypermetric cone into L-domains
The hypermetric cone \HYP_{n+1} is the parameter space of basic Delaunay
polytopes in n-dimensional lattice. The cone \HYP_{n+1} is polyhedral; one
way of seeing this is that modulo image by the covariance map \HYP_{n+1} is a
finite union of L-domains, i.e., of parameter space of full Delaunay
tessellations.
In this paper, we study this partition of the hypermetric cone into
L-domains. In particular, it is proved that the cone \HYP_{n+1} of
hypermetrics on n+1 points contains exactly {1/2}n! principal L-domains. We
give a detailed description of the decomposition of \HYP_{n+1} for n=2,3,4
and a computer result for n=5 (see Table \ref{TableDataHYPn}). Remarkable
properties of the root system are key for the decomposition of
\HYP_5.Comment: 20 pages 2 figures, 2 table
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
Distances on the tropical line determined by two points
Let . Write if is a multiple of
. Two different points and in uniquely
determine a tropical line , passing through them, and stable under
small perturbations. This line is a balanced unrooted semi--labeled tree on
leaves. It is also a metric graph.
If some representatives and of and are the first and second
columns of some real normal idempotent order matrix , we prove that the
tree is described by a matrix , easily obtained from . We also
prove that is caterpillar. We prove that every vertex in
belongs to the tropical linear segment joining and . A vertex, denoted
, closest (w.r.t tropical distance) to exists in . Same for
. The distances between pairs of adjacent vertices in and the
distances \dd(p,pq), \dd(qp,q) and \dd(p,q) are certain entries of the
matrix . In addition, if and are generic, then the tree
is trivalent. The entries of are differences (i.e., sum of principal
diagonal minus sum of secondary diagonal) of order 2 minors of the first two
columns of .Comment: New corrected version. 31 pages and 9 figures. The main result is
theorem 13. This is a generalization of theorem 7 to arbitrary n. Theorem 7
was obtained with A. Jim\'enez; see Arxiv 1205.416
Lossy gossip and composition of metrics
We study the monoid generated by n-by-n distance matrices under tropical (or
min-plus) multiplication. Using the tropical geometry of the orthogonal group,
we prove that this monoid is a finite polyhedral fan of dimension n(n-1)/2, and
we compute the structure of this fan for n up to 5. The monoid captures gossip
among n gossipers over lossy phone lines, and contains the gossip monoid over
ordinary phone lines as a submonoid. We prove several new results about this
submonoid, as well. In particular, we establish a sharp bound on chains of
calls in each of which someone learns something new.Comment: Minor textual edits, final versio
Trees, Tight-Spans and Point Configuration
Tight-spans of metrics were first introduced by Isbell in 1964 and
rediscovered and studied by others, most notably by Dress, who gave them this
name. Subsequently, it was found that tight-spans could be defined for more
general maps, such as directed metrics and distances, and more recently for
diversities. In this paper, we show that all of these tight-spans as well as
some related constructions can be defined in terms of point configurations.
This provides a useful way in which to study these objects in a unified and
systematic way. We also show that by using point configurations we can recover
results concerning one-dimensional tight-spans for all of the maps we consider,
as well as extend these and other results to more general maps such as
symmetric and unsymmetric maps.Comment: 21 pages, 2 figure
Hyperconvexity and Tight Span Theory for Diversities
The tight span, or injective envelope, is an elegant and useful construction
that takes a metric space and returns the smallest hyperconvex space into which
it can be embedded. The concept has stimulated a large body of theory and has
applications to metric classification and data visualisation. Here we introduce
a generalisation of metrics, called diversities, and demonstrate that the rich
theory associated to metric tight spans and hyperconvexity extends to a
seemingly richer theory of diversity tight spans and hyperconvexity.Comment: revised in response to referee comment