414 research outputs found
Stacked polytopes and tight triangulations of manifolds
Tightness of a triangulated manifold is a topological condition, roughly
meaning that any simplexwise linear embedding of the triangulation into
euclidean space is "as convex as possible". It can thus be understood as a
generalization of the concept of convexity. In even dimensions,
super-neighborliness is known to be a purely combinatorial condition which
implies the tightness of a triangulation.
Here we present other sufficient and purely combinatorial conditions which
can be applied to the odd-dimensional case as well. One of the conditions is
that all vertex links are stacked spheres, which implies that the triangulation
is in Walkup's class . We show that in any dimension
\emph{tight-neighborly} triangulations as defined by Lutz, Sulanke and Swartz
are tight.
Furthermore, triangulations with -stacked vertex links and the centrally
symmetric case are discussed.Comment: 28 pages, 2 figure
The complexity of the normal surface solution space
Normal surface theory is a central tool in algorithmic three-dimensional
topology, and the enumeration of vertex normal surfaces is the computational
bottleneck in many important algorithms. However, it is not well understood how
the number of such surfaces grows in relation to the size of the underlying
triangulation. Here we address this problem in both theory and practice. In
theory, we tighten the exponential upper bound substantially; furthermore, we
construct pathological triangulations that prove an exponential bound to be
unavoidable. In practice, we undertake a comprehensive analysis of millions of
triangulations and find that in general the number of vertex normal surfaces is
remarkably small, with strong evidence that our pathological triangulations may
in fact be the worst case scenarios. This analysis is the first of its kind,
and the striking behaviour that we observe has important implications for the
feasibility of topological algorithms in three dimensions.Comment: Extended abstract (i.e., conference-style), 14 pages, 8 figures, 2
tables; v2: added minor clarification
Separation index of graphs and stacked 2-spheres
In 1987, Kalai proved that stacked spheres of dimension are
characterised by the fact that they attain equality in Barnette's celebrated
Lower Bound Theorem. This result does not extend to dimension . In this
article, we give a characterisation of stacked -spheres using what we call
the {\em separation index}. Namely, we show that the separation index of a
triangulated -sphere is maximal if and only if it is stacked. In addition,
we prove that, amongst all -vertex triangulated -spheres, the separation
index is {\em minimised} by some -vertex flag sphere for .
Furthermore, we apply this characterisation of stacked -spheres to settle
the outstanding -dimensional case of the Lutz-Sulanke-Swartz conjecture that
"tight-neighbourly triangulated manifolds are tight". For dimension ,
the conjecture has already been proved by Effenberger following a result of
Novik and Swartz.Comment: Some typos corrected, to appear in "Journal of Combinatorial Theory
A
Combinatorial Seifert fibred spaces with transitive cyclic automorphism group
In combinatorial topology we aim to triangulate manifolds such that their
topological properties are reflected in the combinatorial structure of their
description. Here, we give a combinatorial criterion on when exactly
triangulations of 3-manifolds with transitive cyclic symmetry can be
generalised to an infinite family of such triangulations with similarly strong
combinatorial properties. In particular, we construct triangulations of Seifert
fibred spaces with transitive cyclic symmetry where the symmetry preserves the
fibres and acts non-trivially on the homology of the spaces. The triangulations
include the Brieskorn homology spheres , the lens spaces
and, as a limit case, .Comment: 28 pages, 9 figures. Minor update. To appear in Israel Journal of
Mathematic
Triangulated Manifolds with Few Vertices: Centrally Symmetric Spheres and Products of Spheres
The aim of this paper is to give a survey of the known results concerning
centrally symmetric polytopes, spheres, and manifolds. We further enumerate
nearly neighborly centrally symmetric spheres and centrally symmetric products
of spheres with dihedral or cyclic symmetry on few vertices, and we present an
infinite series of vertex-transitive nearly neighborly centrally symmetric
3-spheres.Comment: 26 pages, 8 figure
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