7,566 research outputs found
Toric cohomological rigidity of simple convex polytopes
A simple convex polytope is \emph{cohomologically rigid} if its
combinatorial structure is determined by the cohomology ring of a quasitoric
manifold over . Not every has this property, but some important
polytopes such as simplices or cubes are known to be cohomologically rigid. In
this article we investigate the cohomological rigidity of polytopes and
establish it for several new classes of polytopes including products of
simplices. Cohomological rigidity of is related to the \emph{bigraded Betti
numbers} of its \emph{Stanley--Reisner ring}, another important invariants
coming from combinatorial commutative algebra.Comment: 18 pages, 1 figure, 2 tables; revised versio
A combinatorial model for computing volumes of flow polytopes
We introduce new families of combinatorial objects whose enumeration computes
volumes of flow polytopes. These objects provide an interpretation, based on
parking functions, of Baldoni and Vergne's generalization of a volume formula
originally due to Lidskii. We recover known flow polytope volume formulas and
prove new volume formulas for flow polytopes that were seemingly
unapproachable. A highlight of our model is an elegant formula for the flow
polytope of a graph we call the caracol graph.
As by-products of our work, we uncover a new triangle of numbers that
interpolates between Catalan numbers and the number of parking functions, we
prove the log-concavity of rows of this triangle along with other sequences
derived from volume computations, and we introduce a new Ehrhart-like
polynomial for flow polytope volume and conjecture product formulas for the
polytopes we consider.Comment: 34 pages, 15 figures. v2: updated after referee reports; includes a
proof of Proposition 8.7. Accepted into Transactions of the AM
Construction and Analysis of Projected Deformed Products
We introduce a deformed product construction for simple polytopes in terms of
lower-triangular block matrix representations. We further show how Gale duality
can be employed for the construction and for the analysis of deformed products
such that specified faces (e.g. all the k-faces) are ``strictly preserved''
under projection. Thus, starting from an arbitrary neighborly simplicial
(d-2)-polytope Q on n-1 vertices we construct a deformed n-cube, whose
projection to the last dcoordinates yields a neighborly cubical d-polytope. As
an extension of thecubical case, we construct matrix representations of
deformed products of(even) polygons (DPPs), which have a projection to d-space
that retains the complete (\lfloor \tfrac{d}{2} \rfloor - 1)-skeleton. In both
cases the combinatorial structure of the images under projection is completely
determined by the neighborly polytope Q: Our analysis provides explicit
combinatorial descriptions. This yields a multitude of combinatorially
different neighborly cubical polytopes and DPPs. As a special case, we obtain
simplified descriptions of the neighborly cubical polytopes of Joswig & Ziegler
(2000) as well as of the ``projected deformed products of polygons'' that were
announced by Ziegler (2004), a family of 4-polytopes whose ``fatness'' gets
arbitrarily close to 9.Comment: 20 pages, 5 figure
One-Point Suspensions and Wreath Products of Polytopes and Spheres
It is known that the suspension of a simplicial complex can be realized with
only one additional point. Suitable iterations of this construction generate
highly symmetric simplicial complexes with various interesting combinatorial
and topological properties. In particular, infinitely many non-PL spheres as
well as contractible simplicial complexes with a vertex-transitive group of
automorphisms can be obtained in this way.Comment: 17 pages, 8 figure
Non-projectability of polytope skeleta
We investigate necessary conditions for the existence of projections of
polytopes that preserve full k-skeleta. More precisely, given the combinatorics
of a polytope and the dimension e of the target space, what are obstructions to
the existence of a geometric realization of a polytope with the given
combinatorial type such that a linear projection to e-space strictly preserves
the k-skeleton. Building on the work of Sanyal (2009), we develop a general
framework to calculate obstructions to the existence of such realizations using
topological combinatorics. Our obstructions take the form of graph colorings
and linear integer programs. We focus on polytopes of product type and
calculate the obstructions for products of polygons, products of simplices, and
wedge products of polytopes. Our results show the limitations of constructions
for the deformed products of polygons of Sanyal & Ziegler (2009) and the wedge
product surfaces of R\"orig & Ziegler (2009) and complement their results.Comment: 18 pages, 2 figure
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