695 research outputs found
Many 2-level polytopes from matroids
The family of 2-level matroids, that is, matroids whose base polytope is
2-level, has been recently studied and characterized by means of combinatorial
properties. 2-level matroids generalize series-parallel graphs, which have been
already successfully analyzed from the enumerative perspective.
We bring to light some structural properties of 2-level matroids and exploit
them for enumerative purposes. Moreover, the counting results are used to show
that the number of combinatorially non-equivalent (n-1)-dimensional 2-level
polytopes is bounded from below by , where
and .Comment: revised version, 19 pages, 7 figure
The universality theorem for neighborly polytopes
In this note, we prove that every open primary basic semialgebraic set is
stably equivalent to the realization space of an even-dimensional neighborly
polytope. This in particular provides the final step for Mn\"ev's proof of the
universality theorem for simplicial polytopes.Comment: 5 pages, 1 figure. Small change
Universality theorems for inscribed polytopes and Delaunay triangulations
We prove that every primary basic semialgebraic set is homotopy equivalent to
the set of inscribed realizations (up to M\"obius transformation) of a
polytope. If the semialgebraic set is moreover open, then, in addition, we
prove that (up to homotopy) it is a retract of the realization space of some
inscribed neighborly (and simplicial) polytope. We also show that all algebraic
extensions of are needed to coordinatize inscribed polytopes.
These statements show that inscribed polytopes exhibit the Mn\"ev universality
phenomenon.
Via stereographic projections, these theorems have a direct translation to
universality theorems for Delaunay subdivisions. In particular, our results
imply that the realizability problem for Delaunay triangulations is
polynomially equivalent to the existential theory of the reals.Comment: 15 pages, 2 figure
Generic and special constructions of pure O-sequences
It is shown that the h-vectors of Stanley-Reisner rings of three classes of
matroids are pure O-sequences. The classes are (a) matroids that are
truncations of other matroids, or more generally of Cohen-Macaulay complexes,
(b) matroids whose dual is (rank + 2)-partite, and (c) matroids of
Cohen-Macaulay type at most five. Consequences for the computational search for
a counterexample to a conjecture of Stanley are discussed.Comment: 16 pages, v2: various small improvements, accepted by Bulletin of the
London Math. Societ
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