72 research outputs found
Combinatorics and Geometry of Transportation Polytopes: An Update
A transportation polytope consists of all multidimensional arrays or tables
of non-negative real numbers that satisfy certain sum conditions on subsets of
the entries. They arise naturally in optimization and statistics, and also have
interest for discrete mathematics because permutation matrices, latin squares,
and magic squares appear naturally as lattice points of these polytopes.
In this paper we survey advances on the understanding of the combinatorics
and geometry of these polyhedra and include some recent unpublished results on
the diameter of graphs of these polytopes. In particular, this is a thirty-year
update on the status of a list of open questions last visited in the 1984 book
by Yemelichev, Kovalev and Kravtsov and the 1986 survey paper of Vlach.Comment: 35 pages, 13 figure
The set of autotopisms of partial Latin squares
Symmetries of a partial Latin square are determined by its autotopism group.
Analogously to the case of Latin squares, given an isotopism , the
cardinality of the set of partial Latin squares which
are invariant under only depends on the conjugacy class of the latter,
or, equivalently, on its cycle structure. In the current paper, the cycle
structures of the set of autotopisms of partial Latin squares are characterized
and several related properties studied. It is also seen that the cycle
structure of determines the possible sizes of the elements of
and the number of those partial Latin squares of this
set with a given size. Finally, it is generalized the traditional notion of
partial Latin square completable to a Latin square.Comment: 20 pages, 4 table
Planar 3-dimensional assignment problems with Monge-like cost arrays
Given an cost array we consider the problem -P3AP
which consists in finding pairwise disjoint permutations
of such that
is minimized. For the case
the planar 3-dimensional assignment problem P3AP results.
Our main result concerns the -P3AP on cost arrays that are layered
Monge arrays. In a layered Monge array all matrices that result
from fixing the third index are Monge matrices. We prove that the -P3AP
and the P3AP remain NP-hard for layered Monge arrays. Furthermore, we show that
in the layered Monge case there always exists an optimal solution of the
-3PAP which can be represented as matrix with bandwidth . This
structural result allows us to provide a dynamic programming algorithm that
solves the -P3AP in polynomial time on layered Monge arrays when is
fixed.Comment: 16 pages, appendix will follow in v
2014 Conference Abstracts: Annual Undergraduate Research Conference at the Interface of Biology and Mathematics
Conference schedule and abstract book for the Sixth Annual Undergraduate Research Conference at the Interface of Biology and Mathematics
Date: November 1-2, 2014Plenary Speakers: Joseph Tien, Associate Professor of Mathematics at The Ohio State University; and Jeremy Smith, Governor\u27s Chair at the University of Tennessee and Director of the University of Tennessee/Oak Ridge National Lab Center for Molecular Biophysic
Facets of the axial three-index assignment polytope
We revisit the facial structure of the axial 3-index assignment polytope. After reviewing known classes of facet-defining inequalities, we present a new class of valid inequalities, and show that they are facets of this polytope. This answers a question posed by Qi and Sun~\cite{QiSun00}. Moreover, we show that we can separate these inequalities in polynomial time
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