1,386 research outputs found
Posets arising as 1-skeleta of simple polytopes, the nonrevisiting path conjecture, and poset topology
Given any polytope and any generic linear functional , one
obtains a directed graph by taking the 1-skeleton of and
orienting each edge from to for .
This paper raises the question of finding sufficient conditions on a polytope
and generic cost vector so that the graph will
not have any directed paths which revisit any face of after departing from
that face. This is in a sense equivalent to the question of finding conditions
on and under which the simplex method for linear programming
will be efficient under all choices of pivot rules. Conditions on and are given which provably yield a corollary of the desired face
nonrevisiting property and which are conjectured to give the desired property
itself. This conjecture is proven for 3-polytopes and for spindles having the
two distinguished vertices as source and sink; this shows that known
counterexamples to the Hirsch Conjecture will not provide counterexamples to
this conjecture.
A part of the proposed set of conditions is that be the
Hasse diagram of a partially ordered set, which is equivalent to requiring non
revisiting of 1-dimensional faces. This opens the door to the usage of
poset-theoretic techniques. This work also leads to a result for simple
polytopes in which is the Hasse diagram of a lattice L that the
order complex of each open interval in L is homotopy equivalent to a ball or a
sphere of some dimension. Applications are given to the weak Bruhat order, the
Tamari lattice, and more generally to the Cambrian lattices, using realizations
of the Hasse diagrams of these posets as 1-skeleta of permutahedra,
associahedra, and generalized associahedra.Comment: new results for 3-polytopes and spindles added; exposition
substantially improved throughou
Computational determination of the largest lattice polytope diameter
A lattice (d, k)-polytope is the convex hull of a set of points in dimension
d whose coordinates are integers between 0 and k. Let {\delta}(d, k) be the
largest diameter over all lattice (d, k)-polytopes. We develop a computational
framework to determine {\delta}(d, k) for small instances. We show that
{\delta}(3, 4) = 7 and {\delta}(3, 5) = 9; that is, we verify for (d, k) = (3,
4) and (3, 5) the conjecture whereby {\delta}(d, k) is at most (k + 1)d/2 and
is achieved, up to translation, by a Minkowski sum of lattice vectors
Computational determination of the largest lattice polytope diameter
A lattice (d, k)-polytope is the convex hull of a set of points in dimension
d whose coordinates are integers between 0 and k. Let {\delta}(d, k) be the
largest diameter over all lattice (d, k)-polytopes. We develop a computational
framework to determine {\delta}(d, k) for small instances. We show that
{\delta}(3, 4) = 7 and {\delta}(3, 5) = 9; that is, we verify for (d, k) = (3,
4) and (3, 5) the conjecture whereby {\delta}(d, k) is at most (k + 1)d/2 and
is achieved, up to translation, by a Minkowski sum of lattice vectors
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
The Gromov Norm of the Product of Two Surfaces
We make an estimation of the value of the Gromov norm of the Cartesian
product of two surfaces. Our method uses a connection between these norms and
the minimal size of triangulations of the products of two polygons. This allows
us to prove that the Gromov norm of this product is between 32 and 52 when both
factors have genus 2. The case of arbitrary genera is easy to deduce form this
one.Comment: The journal version contains an error that invalidates one direction
of the main theorem. The present version contains an erratum, at the end,
explaining thi
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