22 research outputs found
A Class of Hypergraph Arrangements with Shellable Intersection Lattice
AbstractFor every hypergraph onnvertices there is an associated subspace arrangement in Rncalled a hypergraph arrangement. We prove shellability for the intersection lattices of a large class of hypergraph arrangements. This class incorporates all the hypergraph arrangements which were previously shown to have shellable intersection lattices
Hypergraph coloring complexes
AbstractThe aim of this paper is to generalize the notion of the coloring complex of a graph to hypergraphs. We present three different interpretations of those complexes–a purely combinatorial one and two geometric ones. It is shown, that most of the properties, which are known to be true for coloring complexes of graphs, break down in this more general setting, e.g., Cohen–Macaulayness and partitionability. Nevertheless, we are able to provide bounds for the f- and h-vectors of those complexes which yield new bounds on chromatic polynomials of hypergraphs. Moreover, though it is proven that the coloring complex of a hypergraph has a wedge decomposition, we provide an example showing that in general this decomposition is not homotopy equivalent to a wedge of spheres. In addition, we can completely characterize those hypergraphs whose coloring complex is connected
Geometric, Algebraic, and Topological Combinatorics
The 2019 Oberwolfach meeting "Geometric, Algebraic and Topological Combinatorics"
was organized by Gil Kalai (Jerusalem), Isabella Novik (Seattle),
Francisco Santos (Santander), and Volkmar Welker (Marburg). It covered
a wide variety of aspects of Discrete Geometry, Algebraic Combinatorics
with geometric flavor, and Topological Combinatorics. Some of the
highlights of the conference included (1) Karim Adiprasito presented his
very recent proof of the -conjecture for spheres (as a talk and as a "Q\&A"
evening session) (2) Federico Ardila gave an overview on "The geometry of matroids",
including his recent extension with Denham and Huh of previous work of Adiprasito, Huh and Katz
Ehrhart f*-coefficients of polytopal complexes are non-negative integers
The Ehrhart polynomial of an integral polytope counts the number of
integer points in integral dilates of . Ehrhart polynomials of polytopes are
often described in terms of their Ehrhart -vector (aka Ehrhart
-vector), which is the vector of coefficients of with respect to
a certain binomial basis and which coincides with the -vector of a regular
unimodular triangulation of (if one exists). One important result by
Stanley about -vectors of polytopes is that their entries are always
non-negative. However, recent combinatorial applications of Ehrhart theory give
rise to polytopal complexes with -vectors that have negative entries.
In this article we introduce the Ehrhart -vector of polytopes or, more
generally, of polytopal complexes . These are again coefficient vectors of
with respect to a certain binomial basis of the space of polynomials and
they have the property that the -vector of a unimodular simplicial complex
coincides with its -vector. The main result of this article is a counting
interpretation for the -coefficients which implies that -coefficients
of integral polytopal complexes are always non-negative integers. This holds
even if the polytopal complex does not have a unimodular triangulation and if
its -vector does have negative entries. Our main technical tool is a new
partition of the set of lattice points in a simplicial cone into discrete
cones. Further results include a complete characterization of Ehrhart
polynomials of integral partial polytopal complexes and a non-negativity
theorem for the -vectors of rational polytopal complexes.Comment: 19 pages, 1 figur