scholarly journals Every Simplicial Polytope with at Most d + 4 Vertices Is a Quotient of a Neighborly Polytope

1997 ◽  
Vol 18 (4) ◽  
pp. 455-462 ◽  
Author(s):  
U. H. Kortenkamp
Keyword(s):  
Simplicial Polytope ◽  
Neighborly Polytope

scholarly journals NONCROSSING SETS AND A GRASSMANN ASSOCIAHEDRON

2017 ◽  
Vol 5 ◽  
Author(s):  
FRANCISCO SANTOS ◽  
CHRISTIAN STUMP ◽  
VOLKMAR WELKER
Keyword(s):  
Simplicial Complex ◽  
Natural Generalization ◽  
Adjacency Graph ◽  
Simplicial Polytope ◽  
Acyclic Orientation ◽  
Weak Separability ◽  
Initial Ideal ◽  
Alternative Approach ◽  
Invariant Part ◽  
Order Polytope

We study a natural generalization of the noncrossing relation between pairs of elements in$[n]$to$k$-tuples in$[n]$that was first considered by Petersenet al.[J. Algebra324(5) (2010), 951–969]. We give an alternative approach to their result that the flag simplicial complex on$\binom{[n]}{k}$induced by this relation is a regular, unimodular and flag triangulation of the order polytope of the poset given by the product$[k]\times [n-k]$of two chains (also called Gelfand–Tsetlin polytope), and that it is the join of a simplex and a sphere (that is, it is a Gorenstein triangulation). We then observe that this already implies the existence of a flag simplicial polytope generalizing the dual associahedron, whose Stanley–Reisner ideal is an initial ideal of the Grassmann–Plücker ideal, while previous constructions of such a polytope did not guarantee flagness nor reduced to the dual associahedron for$k=2$. On our way we provide general results about order polytopes and their triangulations. We call the simplicial complex thenoncrossing complex, and the polytope derived from it the dualGrassmann associahedron. We extend results of Petersenet al.[J. Algebra324(5) (2010), 951–969] showing that the noncrossing complex and the Grassmann associahedron naturally reflect the relations between Grassmannians with different parameters, in particular the isomorphism$G_{k,n}\cong G_{n-k,n}$. Moreover, our approach allows us to show that the adjacency graph of the noncrossing complex admits a natural acyclic orientation that allows us to define aGrassmann–Tamari orderon maximal noncrossing families. Finally, we look at the precise relation of the noncrossing complex and the weak separability complex of Leclerc and Zelevinsky [Amer. Math. Soc. Transl.181(2) (1998), 85–108]; see also Scott [J. Algebra290(1) (2005), 204–220] among others. We show that the weak separability complex is not only a subcomplex of the noncrossing complex as noted by Petersenet al.[J. Algebra324(5) (2010), 951–969] but actually its cyclically invariant part.

scholarly journals The Mean Width of Circumscribed Random Polytopes

2010 ◽  
Vol 53 (4) ◽  
pp. 614-628 ◽  
Author(s):  
Károly J. Böröczky ◽  
Rolf Schneider
Keyword(s):  
Convex Body ◽  
Asymptotic Formula ◽  
Uniform Distribution ◽  
Lower Bounds ◽  
Upper And Lower Bounds ◽  
Simplicial Polytope ◽  
Mean Width ◽  
The Mean ◽  
The Difference ◽  
Precise Asymptotic

AbstractFor a given convex body K in ℝd, a random polytope K(n) is defined (essentially) as the intersection of n independent closed halfspaces containing K and having an isotropic and (in a specified sense) uniform distribution. We prove upper and lower bounds of optimal orders for the difference of the mean widths of K(n) and K as n tends to infinity. For a simplicial polytope P, a precise asymptotic formula for the difference of the mean widths of P(n) and P is obtained.

scholarly journals Constructing neighborly polytopes and oriented matroids

2012 ◽  
Vol DMTCS Proceedings vol. AR,... (Proceedings) ◽  
Author(s):  
Arnau Padrol
Keyword(s):  
Oriented Matroids ◽  
New Technique ◽  
Oriented Matroid ◽  
Nous Proposons ◽  
International Audience ◽  
New Construction ◽  
A New Technique ◽  
Neighborly Polytope

International audience A $d$-polytope $P$ is neighborly if every subset of $\lfloor\frac{d}{2}\rfloor $vertices is a face of $P$. In 1982, Shemer introduced a sewing construction that allows to add a vertex to a neighborly polytope in such a way as to obtain a new neighborly polytope. With this, he constructed superexponentially many different neighborly polytopes. The concept of neighborliness extends naturally to oriented matroids. Duals of neighborly oriented matroids also have a nice characterization: balanced oriented matroids. In this paper, we generalize Shemer's sewing construction to oriented matroids, providing a simpler proof. Moreover we provide a new technique that allows to construct balanced oriented matroids. In the dual setting, it constructs a neighborly oriented matroid whose contraction at a particular vertex is a prescribed neighborly oriented matroid. We compare the families of polytopes that can be constructed with both methods, and show that the new construction allows to construct many new polytopes. Un $d$-polytope $P$ est $\textit{neighborly}$ si tout sous-ensemble de $\lfloor\frac{d}{2}\rfloor $ sommets forme une face de $P$. En 1982, Shemer a introduit une construction de couture qui permet de rajouter un sommet à un polytope $\textit{neighborly}$ et d'obtenir un nouveau polytope $\textit{neighborly}$. Cette construction lui permet de construire un nombre super-exponentiel de polytopes $\textit{neighborly}$ distincts. Le concept de $\textit{neighborliness}$ s'étend naturellement aux matroïdes orientés. Les duaux de matroïdes orientés $\textit{neighborly}$ ont de plus une belle caractérisation: ce sont les matroïdes orientés équilibrés. Dans cet article, nous généralisons la construction de couture de Shemer aux matroïdes orientés, ce qui en fournit une démonstration plus simple. Par ailleurs, nous proposons une nouvelle technique qui permet de construire matroïdes orientés équilibrés. Dans le cadre dual, on obtient un matroïde $\textit{neighborly}$ dont la contraction à un sommet distinguè est un matroïde $\textit{neighborly}$ prescrit. Nous comparons les familles de polytopes qui peuvent être construites avec ces deux méthodes, et montrons que la nouvelle construction permet de construire plusieurs nouveaux polytopes.

scholarly journals Noncrossing sets and a Graßmannian associahedron

2014 ◽  
Vol DMTCS Proceedings vol. AT,... (Proceedings) ◽  
Author(s):  
Francisco Santos ◽  
Christian Stump ◽  
Volkmar Welker
Keyword(s):  
Simplicial Complex ◽  
Natural Generalization ◽  
Simplicial Polytope ◽  
Weak Separability ◽  
Initial Ideal ◽  
Nous Montrons ◽  
International Audience ◽  
Order Polytope

International audience We study a natural generalization of the noncrossing relation between pairs of elements in $[n]$ to $k$-tuples in $[n]$. We show that the flag simplicial complex on $\binom{[n]}{k}$ induced by this relation is a regular, unimodular and flag triangulation of the order polytope of the poset given by the product $[k] \times [n-k]$ of two chains, and it is the join of a simplex and a sphere (that is, it is a Gorenstein triangulation). This shows the existence of a flag simplicial polytope whose Stanley-Reisner ideal is an initial ideal of the Graßmann-Plücker ideal, while previous constructions of such a polytope did not guaranteed flagness. The simplicial complex and the polytope derived from it naturally reflect the relations between Graßmannians with different parameters, in particular the isomorphism $G_{k,n} \cong G_{n-k,n}$. This simplicial complex is closely related to the weak separability complex introduced by Zelevinsky and Leclerc. Nous étudions une généralisation naturelle de la relation entre les paires d’éléments non-croisés de $[n]$ et les $k$-uplets de $[n]$. Nous montrons que le complexe simplicial de drapeau sur $\binom{[n]}{k}$ induit par cette relation est une triangulation régulière, unimodulaire et de drapeau du polytope d’ordre de l’ensemble partiellement ordonné obtenu par le produit $[k] \times [n-k]$ des deux chaînes, et c’est la jointure d’un simplexe et une sphère (c’est-à-dire qu’elle est une triangulation de Gorenstein). Cela montre l’existence d’un polytope simplicial de drapeau dont l’idéal de Stanley-Reisner est un idéal initial de l’idéal de Graßmann-Plücker, tandis que les constructions précédentes d’un tel polytope ne garantissaient pas la propriété de drapeau. Le complexe simplicial et le polytope qui en découle reflètent naturellement les relations entre les Grassmanniens avec différents paramètres, en particulier l’isomorphisme $G_{k,n} \cong G_{n-k,n}$. Ce complexe simplicial est étroitement lié au complexe de séparabilité faible étudié par Zelevinskyet Leclerc.

A convex 3-complex not simplicially isomorphic to a strictly convex complex

1980 ◽  
Vol 88 (2) ◽  
pp. 299-306 ◽  
Author(s):  
Robert Connelly ◽  
David W. Henderson
Keyword(s):  
Euclidean Space ◽  
Simplicial Complex ◽  
Line Segment ◽  
Boundary Point ◽  
Support Plane ◽  
Simplicial Polytope ◽  
Strictly Convex ◽  
Underlying Space

A set X in euclidean space is convex if the line segment joining any two points of X is in X. If X is convex, every boundary point is on an (n − 1)-plane which contains X in one of its two closed half-spaces. Such a plane is called a support plane for X. A simplicial complex K in is called strictly convex if |K| (the underlying space of K) is convex and if, for every simplex σ in ∂K (the boundary of K) there is a support plane for |K| whose intersection with |K| is precisely σ In this case |K| is often called a simplicial polytope.

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