All Questions
30 questions
9
votes
4
answers
1k
views
Positivity of Ehrhart polynomial coefficients
Are there any results stating that a given family of convex polytopes have Ehrhart polynomials with non-negative coefficients?
What methods are available for proving such a property for some family ...
- 15.8k
8
votes
1
answer
447
views
Do this polyhedron and other set have names?
Updated: My first post had a mistake because I confused in my mind two different but related sets. Hopefully the description below is correct now.
Let $\Lambda$ be a finite set. Let $\Lambda^{(2)}$ be ...
8
votes
1
answer
725
views
Number of simplicial polytopes with a given f-vector
Plenty of very nice literature is available on the characterization of f-vectors of simplicial complexes of diverse sorts (results by Billera, Bjoerner, Kalai, Stanley, among others). I mention, as an ...
- 902
8
votes
1
answer
498
views
Loday's characterization and enumeration of faces of associahedra (Stasheff polytopes)
From "The multiple facets of the associahedra" by Loday:
Let us consider the formal power series
$$f(x) = x+a_1 x^2 +a_2 x^3 + \cdots+ a_n x^{n+1} + \cdots$$
and let
$$ g(x) = x+b_1 x^2 + ...
- 10.5k
8
votes
1
answer
231
views
Classifying two-faces of four-polytopes
Motivation: This question is related to my study of hyperbolic Coxeter polytopes. In general, if one put some restrictions on the type of their dihedral angles (say, all dihedral angles are equal to $\...
- 1,268
7
votes
2
answers
789
views
"MultiCatalan numbers"
Could anyone provide a reference for the following (sort of) generalization of Catalan numbers: the multinomial coefficient
$$
\binom{2k_1+3k_2+4k_3+...}{k_1+2k_2+3k_3+...,k_1,k_2,k_3,...}
$$
is ...
- 18.4k
7
votes
1
answer
756
views
Generalized permutahedron and random polytopes
The Birkhoff polytope $B_n$ is defined as the convex hull of the set of permutation matrices, which gives us the set of doubly stochastic matrices. A concept which is intimately related is that of the ...
- 407
7
votes
1
answer
299
views
Lipschitz-continuity of convex polytopes under the Hausdorff metric
Recently, I proved the following Lipschitz-continuity like result for convex polytopes:
Let $A\in\mathbb R^{m\times n}$ and $b,b'\in\mathbb R^m$ be given such that $\{x\,:\,Ax\leq 0\}=\{0\}$ (which ...
6
votes
2
answers
2k
views
Birkhoff's theorem about doubly stochastic matrices
Birkhoff's theorem states:
The set of $n \times n$ doubly stochastic matrices is a convex set whose extreme points are the permutation matrices
This theorem seems to be commonly attributed to ...
- 28.6k
6
votes
1
answer
237
views
Some polytopes in $\mathbb R^n$ whose vertices have coordinates 1, -1 or 0
Let $n$ and $k$ be positive integers with $k\leq n$.
Let $P(n,k)$ be the convex hull in $\mathbb R^n$ of the $2^k {n \choose k}$ vectors whose exactly $k$ coordinates belong to $\{\pm 1\}$ all the ...
- 838
6
votes
1
answer
185
views
Maximizing ratio volume/diameter^n by an affinity
Suppose we have a convex compact body $D\subset \mathbb R^n$. We can try to apply affine transformation keeping the volume and decreasing the diameter of $D$.
It is clear that there is a constant $\...
- 5,053
5
votes
1
answer
316
views
Coefficients of Ehrhart polynomials, in the binomial-coefficient basis
Let $P$ be the convex hull of a finite set of points in $\mathbb Z^d$, and $p(n) = \#\{nP \cap \mathbb Z^d\}$ be its Ehrhart polynomial, which is also the Hilbert polynomial of the corresponding ...
- 27.8k
5
votes
0
answers
474
views
Convex polytopes as "products" of lower dimensional polytopes of the same family
This MO-Q details the sense in which an associahedron is a product of lower dimensional associahedra, and this MSE-Q indicates the same is true for permutohedra.
Is there a reference which classifies ...
- 10.5k
4
votes
2
answers
173
views
4-polytopes with only one kind of regular facet
Is there a neat way to show (or a reference that already proves) that
the 4-cube is the only convex 4-polytope in which all facets are regular 3-cubes?
the 24-cell is the only convex 4-polytope in ...
- 13.6k
4
votes
0
answers
130
views
"Baues poset" of shellings of simplicial polytope?
Let me start with some background I want to use as analogy.
Consider a (convex) polytope $P$ and its set of triangulations. Among all the triangulations, a well behaved subset are the regular ones: ...
- 24.2k
4
votes
0
answers
158
views
Reference for the notion of polyhedra "degenerations"
Let $P$ be a convex polyhedron and let $P(t)$ be a continuous deformation thereof, such that:
a) $P(0)=P$;
b) for all $t\in[0;1)$ the polyhedron $P(t)$ is strongly combinatorially equivalent to $P$ (...
- 3,513
3
votes
2
answers
270
views
Formalization (and background) of a formula, concering the integral points of a polygon.
I have recently become aware of the following neat statement.
Consider a convex polygon $P$ in the real plane with integral vertices. If we associate with every integral point $(a,b)$ the monomial $x^...
- 3,513
3
votes
1
answer
445
views
Dehn-Sommerville relations for $\Delta$-complexes
Let $M$ be a closed, triangulated manifold of dimension $m$ and $K(M)$ be its triangulation. Let $f_i$ denote the number of $i$-simplices of $K(M)$. As proved by Klee the face numbers satisfy the ...
- 1,017
3
votes
1
answer
271
views
The facial structure of the convex hull of a family of characteristic functions
Let $S$ be a finite set and let $\mathcal{A} \subset\mathcal{P}(S)$ be a family of subsets of $S$. Consider the convex polytope spanned by the characteristic functions of members of $\mathcal{A}$ :
$$...
- 60.5k
3
votes
0
answers
179
views
Polytope algebra and toric vareties
Let $\Pi$ denote the McMullen polytope algebra (over the field of rationals $\mathbb{Q}$) generated by convex polytopes with rational vertices in $\mathbb{R}^n$.
For a simple polytope $P$ let us ...
- 21.8k
3
votes
0
answers
102
views
The ring generated by a convex polytope and its faces
Let $V=\Bbb R^n$. Morelli defined the (commutative unital) ring $L(V)$ to be the additive group generated by the indicator functions of convex polytopes in $V$ with multiplication induced by Minkowski ...
- 3,079
3
votes
0
answers
219
views
First to note/document the relation between permutohedra and multiplicative inversion
The relation between the refined face numbers of the permutohedra and the formal series expansion of the reciprocal of a function (exponential generating function, formal Taylor series) is given in ...
- 10.5k
2
votes
1
answer
295
views
Are orbit polytopes of rotation subgroup of Coxeter group combinatorially equivalent?
Suppose that $G\subset O(d)$ is a finite reflection (finite Coxeter) group. For any $v\in \mathbb{R}^d$ which is not fixed by any non-trivial $g\in G$, one can consider the orbit polytope (Coxeter) ...
- 439
2
votes
0
answers
252
views
Understanding the geometry of $H_{n}=\{\vec{x} \in [-N,N]^n:\sum_{i=1}^n x_i = 0\}$
I am not an expert in convex geometry but if we define $a_i \sim \mathcal{U}([-N,N])$ where $[-N,N] \subset \mathbb{R}$ and $S_n = \sum_{i=1}^n a_i$ I suspect that for arbitrary $N \in [1, \infty) $:
...
- 3,871
1
vote
1
answer
103
views
A source for $01$-polytopes
Can you recommend any books or survey articles on $01$-polytopes, thats is, polytopes with vertices in $\{0,1\}^n$?
I am less interested in random $01$-polytopes, but more in the combinatorial ...
- 13.6k
1
vote
1
answer
106
views
Linear relations between volume of a polytope and its faces
Let $P$ be a polytope. Is anything known about the set of linear relations that hold between the volumes of the (not-necessarily proper) faces of $P$ as $P$ “varies slightly”? By varies slightly I ...
- 3,079
1
vote
0
answers
154
views
Volume of a polytope as its degenerates to be lower dimensional
Consider a polytope $P$ defined by the usual inequalities $A\mathbf{x}\leq \mathbf{b}$; let me assume that $P$ is not contained in a proper subspace. A result which I believe to true, but am not ...
- 44.7k
1
vote
0
answers
49
views
Realizing 0/1-polytopes with shortest possible edge lengths
Has there been something written about the following question?
Question: Given a 0/1-polytope, what is the shortest edge lengths with which this polytope can be realized as a 0/1-polytope.
The ...
- 13.6k
1
vote
0
answers
193
views
Lattice-point enumeration question involving linear combinations of matrices
I would like to know some references to learn more about an answer to this question, if there are any references:
Let $A_1, \dots , A_m$ and $B$ be $n\times n$ symmetric matrices. Let $$S = \{(x_1, \...
- 170
0
votes
1
answer
108
views
If $C_1\subseteq C_2$ are two closed convex cones that are pointed with $\partial C_1\subseteq \partial C_2$ then is $C_1=C_2$?
Let $C_1$ and $C_2$ be two proper full dimensional closed convex cones in $\mathbb{R}^n$ that are pointed. Suppose that $C_1\subseteq C_2$ and that the boundary of $C_1$ is contained in the boundary ...
