358 research outputs found
Local Covering Optimality of Lattices: Leech Lattice versus Root Lattice E8
We show that the Leech lattice gives a sphere covering which is locally least
dense among lattice coverings. We show that a similar result is false for the
root lattice E8. For this we construct a less dense covering lattice whose
Delone subdivision has a common refinement with the Delone subdivision of E8.
The new lattice yields a sphere covering which is more than 12% less dense than
the formerly best known given by the lattice A8*. Currently, the Leech lattice
is the first and only known example of a locally optimal lattice covering
having a non-simplicial Delone subdivision. We hereby in particular answer a
question of Dickson posed in 1968. By showing that the Leech lattice is rigid
our answer is even strongest possible in a sense.Comment: 13 pages; (v2) major revision: proof of rigidity corrected, full
discussion of E8-case included, src of (v3) contains MAGMA program, (v4) some
correction
Higher dimensional cluster combinatorics and representation theory
Higher Auslander algebras were introduced by Iyama generalizing classical
concepts from representation theory of finite dimensional algebras. Recently
these higher analogues of classical representation theory have been
increasingly studied. Cyclic polytopes are classical objects of study in convex
geometry. In particular, their triangulations have been studied with a view
towards generalizing the rich combinatorial structure of triangulations of
polygons. In this paper, we demonstrate a connection between these two
seemingly unrelated subjects.
We study triangulations of even-dimensional cyclic polytopes and tilting
modules for higher Auslander algebras of linearly oriented type A which are
summands of the cluster tilting module. We show that such tilting modules
correspond bijectively to triangulations. Moreover mutations of tilting modules
correspond to bistellar flips of triangulations.
For any d-representation finite algebra we introduce a certain d-dimensional
cluster category and study its cluster tilting objects. For higher Auslander
algebras of linearly oriented type A we obtain a similar correspondence between
cluster tilting objects and triangulations of a certain cyclic polytope.
Finally we study certain functions on generalized laminations in cyclic
polytopes, and show that they satisfy analogues of tropical cluster exchange
relations. Moreover we observe that the terms of these exchange relations are
closely related to the terms occuring in the mutation of cluster tilting
objects.Comment: 41 pages. v4: minor corrections throughout the pape
Asymptotically efficient triangulations of the d-cube
Let and be polytopes, the first of "low" dimension and the second of
"high" dimension. We show how to triangulate the product
efficiently (i.e., with few simplices) starting with a given triangulation of
. Our method has a computational part, where we need to compute an efficient
triangulation of , for a (small) natural number of our
choice. denotes the -simplex.
Our procedure can be applied to obtain (asymptotically) efficient
triangulations of the cube : We decompose , for
a small . Then we recursively assume we have obtained an efficient
triangulation of the second factor and use our method to triangulate the
product. The outcome is that using and , we can triangulate
with simplices, instead of the achievable
before.Comment: 19 pages, 6 figures. Only minor changes from previous versions, some
suggested by anonymous referees. Paper accepted in "Discrete and
Computational Geometry
Cubulations, immersions, mappability and a problem of Habegger
The aim of this paper (inspired from a problem of Habegger) is to describe
the set of cubical decompositions of compact manifolds mod out by a set of
combinatorial moves analogous to the bistellar moves considered by Pachner,
which we call bubble moves. One constructs a surjection from this set onto the
the bordism group of codimension one immersions in the manifold. The connected
sums of manifolds and immersions induce multiplicative structures which are
respected by this surjection. We prove that those cubulations which map
combinatorially into the standard decomposition of for large enough
(called mappable), are equivalent. Finally we classify the cubulations of
the 2-sphere.Comment: Revised version, Ann.Sci.Ecole Norm. Sup. (to appear
Recent progress on the combinatorial diameter of polytopes and simplicial complexes
The Hirsch conjecture, posed in 1957, stated that the graph of a
-dimensional polytope or polyhedron with facets cannot have diameter
greater than . The conjecture itself has been disproved, but what we
know about the underlying question is quite scarce. Most notably, no polynomial
upper bound is known for the diameters that were conjectured to be linear. In
contrast, no polyhedron violating the conjecture by more than 25% is known.
This paper reviews several recent attempts and progress on the question. Some
work in the world of polyhedra or (more often) bounded polytopes, but some try
to shed light on the question by generalizing it to simplicial complexes. In
particular, we include here our recent and previously unpublished proof that
the maximum diameter of arbitrary simplicial complexes is in and
we summarize the main ideas in the polymath 3 project, a web-based collective
effort trying to prove an upper bound of type nd for the diameters of polyhedra
and of more general objects (including, e. g., simplicial manifolds).Comment: 34 pages. This paper supersedes one cited as "On the maximum diameter
of simplicial complexes and abstractions of them, in preparation
Triangulations
The earliest work in topology was often based on explicit combinatorial models – usually triangulations – for the spaces being studied. Although algebraic methods in topology gradually replaced combinatorial ones in the mid-1900s, the emergence of computers later revitalized the study of triangulations. By now there are several distinct mathematical communities actively doing work on different aspects of triangulations. The goal of this workshop was to bring the researchers from these various communities together to stimulate interaction and to benefit from the exchange of ideas and methods
Geometry of Log-Concave Density Estimation
Shape-constrained density estimation is an important topic in mathematical
statistics. We focus on densities on that are log-concave, and
we study geometric properties of the maximum likelihood estimator (MLE) for
weighted samples. Cule, Samworth, and Stewart showed that the logarithm of the
optimal log-concave density is piecewise linear and supported on a regular
subdivision of the samples. This defines a map from the space of weights to the
set of regular subdivisions of the samples, i.e. the face poset of their
secondary polytope. We prove that this map is surjective. In fact, every
regular subdivision arises in the MLE for some set of weights with positive
probability, but coarser subdivisions appear to be more likely to arise than
finer ones. To quantify these results, we introduce a continuous version of the
secondary polytope, whose dual we name the Samworth body. This article
establishes a new link between geometric combinatorics and nonparametric
statistics, and it suggests numerous open problems.Comment: 22 pages, 3 figure
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