All Questions
Tagged with co.combinatorics discrete-geometry
615 questions
9
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0
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144
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Which polytopes have compact realization spaces?
Let $P\subset\Bbb R^d$ be a convex polytope.
Its reduced realization space is the space of all combinatorially equivalent polytopes modulo projective transformations.
I am interested in polytopes for ...
- 13.6k
1
vote
1
answer
99
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Is there any known upper bound for the local crossing number of a graph drawing in the plane?
The local crossing number ${\rm LCR(G)}$ of a graph $G$ is defined as the least nonnegative integer $k$ such that the graph has a $k$-planar drawing. In other words, it is the smallest possible number ...
- 1,190
1
vote
0
answers
51
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Coarse-graining a hypergraph
$\DeclareMathOperator{\poly}{\mathrm{poly}}$I have asked this question on math.SE here, but couldn't get a satisfactory answer. I have also asked a related question on math overflow here, but haven't ...
- 277
-2
votes
1
answer
142
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Solution to Erdos-Ulam problem [closed]
I have solved the Erdos-Ulam problem (see link) and can construct a set that satisfies the conditions (dense in R2 with all interpoint distances rational). I have expanded the solution from two ...
26
votes
0
answers
512
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A non-self-intersecting unit side length polygon in a unit square has odd number of sides unless it is the square itself
This is the same question as here in SE.
I have a conjecture, it is like this:
Suppose there is a non-self-intersecting polygon lies inside a closed square of length $1$. The polygon has every side ...
- 843
4
votes
0
answers
90
views
Definition of Loop in an Oriented Matroid
I had posted this on Stackexchange because I don't believe this is a particlarly difficult question, but there were no answers, so I'm posting it on here now.
I just had a quick question about the ...
- 61
0
votes
0
answers
128
views
The smallest dihedral angle of convex polyhedrons
Given a set of points $\{x_{k}\}_{k=0}^{m} \subset \mathbb{R}^n$, is it always possible to find a constant $c=c(m,n)>0$, depending only on the dimension $n$ and the number $m$, such that, after ...
- 705
0
votes
0
answers
98
views
Number of tetrahedra inside a sphere with boundary A
I understand, that there are some combinatorial problems which are not yet solved regarding gluing triangulations in 3D. At least last time I checked, it was not yet known exactly how many ...
- 183
0
votes
0
answers
176
views
How to find a configuration of lines
In $\mathbb{R}^3$, can anyone help find a configuration of 5 lines such that the minimum of the smallest semi-axis lengths of the ellipsoid $ \mathbf{x}^T \mathbf{A} \mathbf{x} = 1 $, where $\mathbf{A}...
- 61
4
votes
0
answers
66
views
Convergence of graph geodesics to geodesics on metric spaces
Let $(X,d)$ be a compact length space metric space $\mathbb{X}_{\delta}$ be a $\delta$-packing on $X$ and, for every $k\in \mathbb{N}_+$, let $G_{k,\delta}=(\mathbb{X}_{\delta},\mathcal{E}_k,W_k)$ ...
- 364
2
votes
1
answer
754
views
On a combinatorial inequality
Is it true that
\begin{gather}
\min\left(\lambda_{\min}(M_{12}), \lambda_{\min}(M_{13}), \lambda_{\min}(M_{14}), \lambda_{\min}(M_{15}), \lambda_{\min}(M_{23}), \\ \lambda_{\min}(M_{24}), \lambda_{\...
- 178
0
votes
1
answer
98
views
Chromatic tiling complexity and the chromatic number conjecture
Let $T$ be a finite set of tiles in $\mathbb{R}^d$. A tiling of $\mathbb{R}^d$ by $T$ is a collection of disjoint translates of tiles in $T$ whose union is $\mathbb{R}^d$. A tiling is $k$-chromatic if ...
3
votes
1
answer
237
views
Find the number of triangles in plane
Let $S$ be a set of $n$ points in the plane in general position. Each 3 points of S span a triangle. Total number of triangles spanned by S:
$$\binom{n}{3}=\frac{n(n-1)(n-2)}{6}=\frac{1}{6} n^3-O(n^2 )...
- 214
4
votes
1
answer
356
views
Left and right halves of convex curve
Let $S$ be a set of $n$ points in the plane in general position (no 3 on a line), $n$ even.
A halving line is a line through $2$ points of $S$ that partitions $S$ into 2 equal parts ($(n-2)/2$ points ...
- 214
3
votes
0
answers
110
views
How many Tverberg partition are in cloud of points? [closed]
Tverberg's Theorem: A collection of $(d+1)(r-1) +1$ points in $\mathbb{R}^d$ can always be partitioned into $r$ parts whose convex hulls intersect.
For example, $d=2$, $r=3$, 7 points:
Let $p_1, p_2,...
- 214
2
votes
0
answers
318
views
What's the number of facets of a $d$-dimensional cyclic polytope?
A face of a convex polytope $P$ is defined as
$P$ itself, or
a subset of $P$ of the form $P\cap h$, where $h$ is a hyperplane such that $P$ is fully contained in one of the closed half-spaces ...
- 43
6
votes
1
answer
366
views
An arrangement of hyperplanes [closed]
An arrangement of hyperplanes in $\mathbb{R}^d$ is simple if the hyperplanes are in general position (for every $1\leq k\leq d+1$, the intersection of $k$ hyperplanes is $(d-k)$-dimensional).
My ...
- 214
5
votes
1
answer
355
views
How do you traverse a rectangular grid of points while turning as little as possible?
Suppose I have a lattice grid of $m \times n$ points in the plane, with $m\leq n$. I want to traverse this grid in such a way as to minimize the total amount of turning that occurs. I am pretty sure ...
- 4,049
1
vote
1
answer
115
views
Bounds on lengths of boxes in bounded-degree box graphs
$\DeclareMathOperator{\box}{\operatorname{box}}$$\DeclareMathOperator{\cub}{\operatorname{cub}}$
This is a follow up and an extension of another question I asked recently.
A box graph is a graph ...
- 277
1
vote
1
answer
194
views
Bounds on lengths of intervals in bounded-degree interval graphs
A graph is said to be an interval graph if its vertices can be associated with (closed) intervals on the real line $\mathbb R$ and there is an edge between two vertices if and only if the ...
- 277
1
vote
1
answer
141
views
Covering a bounded degree graph with subgraphs of bounded sizes
Let $G$ be a connected graph on $n$ vertices with maximum degree $\Delta \ge 2$. Let $\mathcal G = \{G_1,G_2,\ldots\}$ be a collection of subgraphs of $G$ such that every edge of $G$ is contained in ...
- 277
3
votes
1
answer
605
views
Matryoshka doll problem
Notation: We fix some integer $d \geq 1$ and $N \geq 2$. We use $[m, n]$ to denote the set of integers $\{m, \dots, n\}$, and $\mathbb L_N := [1, N]^d$ to denote the set $\{1, \dots, N\}^d \subset \...
- 6,155
1
vote
0
answers
76
views
Shellable non-pseudomanifolds with dimension greater than 2
Shellability of simplicial balls and spheres (simplicial complexes whose geometric realizations are homeomorphic to balls and spheres) has been studied quite extensively. There are many explicit ...
1
vote
0
answers
42
views
What lower bounds are known for pair crossing number and related questions in multigraphs?
So in terms of crossing number https://arxiv.org/pdf/1808.10480 gives a lower bound of $O(e^{2.5}/n^{1.5})$ for multigraphs with no face of length 2 with no node contained inside.
What do we know ...
- 111
1
vote
0
answers
67
views
Conjecture on the increasing efficiency of the shortest minimum-link polygonal chains covering any grids of the form $\{0,1,2\}^k$ as $k$ grows
From the well-known Nine dots problem, we know that we need a polygonal chain with at least $4$ edges to connect the $9$ points of the planar grid $G_{3,2}:=\{\{0, 1, 2\} \times \{0, 1, 2\}\} \subset \...
- 1,451
0
votes
0
answers
82
views
On 'Bisecting sections' of 3D convex bodies
Following shadows and planar sections, we ask about bisecting sections. This post also continues Convex planar regions with all area bisectors having equal length and A claim on the concurrency of ...
- 5,979
6
votes
1
answer
127
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Convex planar regions with all area bisectors having equal length
Following A claim on the concurrency of area bisectors of planar convex regions, let me record a couple of simple queries.
An area bisector (perimeter bisector) of a planar convex region is a chord ...
- 5,979
6
votes
1
answer
400
views
Szemerédi-Trotter theorem for planes and lines
The Szemerédi-Trotter theorem states:
Theorem Let $P$ be a set of $m$ lines in $\mathbb R^2$ and let $L$ be a set of $n$ points in $\mathbb R^2$. Then
$$\#\{(p,\ell)\in P\times L:p\in\ell\}\lesssim (...
- 3,054
6
votes
1
answer
95
views
Minimum area of the symmetric difference of odd number of translated copies of a unit circle $C$
Let $C$ be the unit circle in a plane. Take an odd number $n$ of translated copies of $C$ and take their symmetric difference $D$. Is it true that the area of $D$ should be at least that of $C$?
If $C$...
- 83
5
votes
0
answers
145
views
Are there convex polyhedrons that can be cut into mutually congruent connected pieces only if pieces are non-convex?
This is the 3D (and higher D) version of A claim on partitioning a convex planar region into congruent pieces
Is there a 3D convex polyhedral solid that can be cut into 2 mutually congruent non-...
- 5,979
2
votes
0
answers
51
views
Convex polygons that can be cut into sets of m mutually congruent convex pieces in exactly n ways
General question: Given two integers m and n, to find a convex polygonal region that can be cut into sets of m mutually congruent convex pieces in exactly n ways - the shape of pieces in each set ...
- 5,979
2
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0
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62
views
On convex polygons that can be cut into convex and mutually congruent pieces in exactly one way
Observations: any thin isosceles triangle has exactly 1 partition into 2 congruent pieces - only 1 line, bisector of its apex, does it.
By attaching a right triangle with base 1 and altitude 2 to an ...
- 5,979
2
votes
2
answers
227
views
On cutting tetrahedrons into mutually congruent pieces
Simple observations: A regular tetrahedron can be cut into 2 mutually congruent pieces (in 3 obvious ways which are all basically the same way, giving one and same pair of congruent pieces). The ...
- 5,979
1
vote
1
answer
178
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Inside-out dissections of solids -2
We record some general questions based on
Inside-out dissections of solids
Inside-out dissections of a cube
Can every convex polyhedral solid be inside-out dissected to a congruent polyhedral solid?...
- 5,979
1
vote
0
answers
93
views
Inside-out dissections of a cube
Ref:
Inside-out polygonal dissections
Inside-out dissections of solids
Definitions: A polygon P has an inside-out dissection into another polygon P' if P′ is congruent to P, and the perimeter of P ...
- 5,979
1
vote
0
answers
194
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'Imperfect' squarings of a square
Ref: https://en.wikipedia.org/wiki/Squaring_the_square
This is a planar version of the question at Cubing the cube - as 'perfectly' as possible.
Question: How does one cut a square into the ...
- 5,979
0
votes
0
answers
79
views
Are there triangles that can be cut into 7 mutually congruent connected polygons?
First question below had appeared in a note at Triangles that can be cut into mutually congruent and non-convex polygons
Following the results of Beeson quoted in the answer at Subdivision of ...
- 5,979
1
vote
1
answer
114
views
Removing a face from 4-connected planar graph
After removing a face (vertices along with edges) of a 4-connected planar graph, is the remaining graph 4-connected? Alternatively under what conditions is this true?
- 129
0
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0
answers
70
views
VC dimension of full-dimensional closed polyhedral cone in $\mathbb R^d$
Consider a fixed set of vectors $\{x_i\}_{i\in[n]}$ in $\mathbb R^d$ and closed polyhedral cone $C = \{w \in \mathbb R^d : w^\top x_i \geq 0, \forall i \in [n]\}$ with full dimension i.e. $C$ contains ...
- 1
2
votes
1
answer
137
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Reconstructing a matroid by its minors
Proposition 3.1.27 in Oxley's Matroid Theory says that given a matroid $M$ and an element $e\in E(M)$ such that $e$ is not a loop or a coloop, the pair $(M/e, M\setminus e)$ uniquely determines $M$. ...
- 61
3
votes
0
answers
224
views
A weight formula for subgraphs of $K_n$ and log-concavity of nested binomial coefficients
Nested binomials Let $t,d$ be positive integers and $n$ a parameter. The degree $td$ rational polynomial
$p_{t,d}(n)={{ n \choose t} \choose d}$ obviously takes integral values for integral $n$ (not ...
- 1,362
2
votes
1
answer
209
views
Do the dual graphs of hyperplane arrangements admit Hamiltonian paths?
Consider a simple hyperplane arrangement $H_1,\cdots,H_n$ in the Euclidean space $\mathbb{R}^d$. By "simple" we mean any $k$ hyperplanes in $\{H_1,\cdots,H_n\}$ intersect in codimension $k$. ...
- 3,187
4
votes
0
answers
138
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Hyponontiling Wang tiles
Call a finite collection of tiles that can tile the plane if we have to use each tile at least once tiling.
Is there a collection of at least 3 tiles that is not tiling, but such that after removing ...
- 18.7k
3
votes
1
answer
855
views
Infinite dimensional lattice for integers and the Riemann hypothesis?
It is known that for each finite set of primes $p$ we have: $\log(p)$ are linear independent over the rational numbers.
We have $\log(ab) = \log(a)+\log(b)$ and $\log(n) = \sum_{p |n}v_p(n) \log(p)$.
...
- 3,064
2
votes
1
answer
184
views
Estimating ${\left(\sum_{i=j}^k {x_i}\right)^2} \times \left\lvert\sum_{i=j}^k {a_i}\right\rvert$
Given two sets; $X = \{x_i : x_i \geq 0; i \in [\sqrt{n}]\}$ and $A = \{a_i : |a_i| \leq 1; i \in [\sqrt{n}]\}$ of size $n^{\frac{1}{2}}$ each, with the following properties
\begin{equation}\label{...
- 23
2
votes
1
answer
106
views
Exhaustive list of small graphs for which $\frac{\alpha(G)\omega(G)}{n}$ is small?
I am looking for a list of small graphs (say on less than 10 vertices) for which the parameter $p(G) = \frac{\alpha(G) \omega(G)}{n}$ is small. Here $\alpha(G)$ and $\omega(G)$ is the size of the ...
- 129
5
votes
1
answer
213
views
Given a 3-connected graph $G$, is there an edge $e$ so that both $G-e$ and $G/e$ are still 3-connected?
Let $G$ be a 3-connected (simple) graph other than $K_4$. In Diestel's "Graph Theory" Section 3.2 we find
Lemma 3.2.2. There is an edge $e$ so that $G\mathbin{\dot-}e$ is still 3-connected (...
- 13.6k
9
votes
0
answers
144
views
How many simplicial spheres with $n$ vertices and $N$ facets?
Let $s_d(n,N)$ be the number of different $d$-dimensional simplicial spheres on $n$ labelled vertices and $N$ facets (= $d$-simplices). I am in search for the best know upper bounds, especially for $d\...
- 13.6k
2
votes
0
answers
87
views
Computationally decomposing a complete geometric graph into forests of stars
I'm working on the following problem:
I would like to see if it possible to decompose a complete geometric graph on $8$ vertices into $5$ planar star-forests. As doing this by hand was hopeless, I ...
- 21
4
votes
0
answers
175
views
Can a square be partitioned into mutually non-congruent triangles all of same area and perimeter?
It is known that the plane cannot be tiled by pair-wise non-congruent triangles all having same area and same perimeter (https://arxiv.org/abs/1711.04504).
Question: Can a square be partitioned into ...
- 5,979