# Tagged Questions

**26**

votes

**2**answers

853 views

### Bodies of constant width?

In two-dimensional case one can generalize figures of constant width as figures which can rotate in a covex polygon.
Here is one example which can be used to drill triangular holes:
I would like to ...

**4**

votes

**4**answers

908 views

### Delaunay triangulations and convex hulls

This is a reference request.
I have the impression that those who work in computational geometry are accustomed to the following. You have some locally finite set of sites in $\mathbb{R}^n$ and you ...

**24**

votes

**2**answers

684 views

### chromatic number of the hyperbolic plane

A notorious problem in combinatorics is the following:
If we color $\mathbb{R}^2$ so that no pair of points at unit distance get the same color, what is the fewest number of colors required?
This ...

**3**

votes

**0**answers

235 views

### Coloring $\mathbb{Z}^k$ and a fixed point theorem

This is potentially another approach to this question. I put it as an update there, but perhaps it would be better to post it separately. If we color $\mathbb{Z}^k$ with the $\ell_\infty$ metric in ...

**6**

votes

**3**answers

248 views

### Minimum separating subdivision in Plane

Hi
I was thinking about the following problem:
Given a planar Graph embedded in the plane and a set of points $P$ contained in the faces (no face contains more than one point) I want to determine ...

**1**

vote

**1**answer

233 views

### Characteristics of locally triangle-free graph

Hi
I am given a triangulation $T $ of a set of points $S $ in the plane and a disk $D$ which doesn't contain any triangle. If I now look at the subgraph $G(V,E)$ of $T $ whose vertices are the points ...

**9**

votes

**1**answer

952 views

### Numbers of intersection points and lines

Hello,
I don't know if this question has already been posted, I have made a little search with keywords and did not found it, sorry if I missed anything.
Is it possible to characterize the set of ...

**17**

votes

**2**answers

783 views

### Placing points on a sphere so that no 3 lie close to the same plane

Motivation
I am working with arbitrary parallelopiped tilings given by projection from a higher dimensional space. The collection of tiles, and some properties of the higher dimensional space are ...

**8**

votes

**2**answers

353 views

### Limit shape for fixed-perimeter lattice polygons

Let $P$ be a simple polygon defined by $n$ unit-length segments
connecting lattice points of $\mathbb{Z}^2$.
I have two operations that preserve the perimeter of $P$.
The first is the "pop" of a ...

**8**

votes

**7**answers

762 views

### Omniscient bots gathering on $\mathbb{Z}^2$

There are $N=n^2$ "bots" on distinct integer lattice points in the plane.
Each knows the positions $p_i$ of all bots, and each has unlimited (private) memory.
Each executes the same algorithm ...

**1**

vote

**0**answers

137 views

### When is the conical hull of a finite set of vectors a subset of the space? (and tilings)

Consider a hypercube in n-dimensions, and take some projection down to an m-dimensional subspace. Now take all vertices and m-1 dimensional facets visible from some direction outside the projection. ...

**1**

vote

**1**answer

619 views

### Applications of ham sandwich type results. References? A general principle?

Lately there has been a lot of interest on applications of the ham sandwich theorem and related results. There is a bunch of lecture notes and surveys that touch upon the subject. I dont know of any ...

**10**

votes

**3**answers

950 views

### A curious generalization of Helly's theorem

Here is a curious conjectural extension of Helly's theorem.
It may follow (if true) from a useful theorem of the kind asked in this MO question:
Conjecture: Let ${\cal F}=P_1,P_2,\dots,P_m$ be a ...

**8**

votes

**1**answer

728 views

### A Desirable Extension of the Nerve Theorem

Backgroud
The Nerve Theorem (see nLab;) asserts that given a finite collection $\cal K$ of compact sets with the property that all non empty intersections of sets in the family are homotopically ...

**-1**

votes

**1**answer

234 views

### Walks that cannot hit the boundary

I've casually proved, as application of some ideas that I am developing, a result that might be of interest in itself. I am completely new in this field and then I would like to ask your help to ...

**4**

votes

**2**answers

597 views

### A Brouwer fixed point theorem on finite sets

I have casually almost (i.e. up to details that shoud work) proved the following discrete version of Brouwer's fixed point theorem. I should have obtained this result as a corollary of quite ...

**11**

votes

**2**answers

2k views

### Weighted area of a Voronoi cell

Let $X = \{ x_1,\dots,x_n\} $ denote a set of $n$ points in the unit square $S = [0,1]\times[0,1]$, and let $w = \{w_1,\dots,w_n\}$ denote a set of weights corresponding to the $n$ points in $X$. ...

**10**

votes

**1**answer

370 views

### Chord arrangement that avoids confining small or large disks

This question is
These two questions are two-dimensional variations on this recent MO question,
"Threading pinholes in the wall of cylinder to pass through an internal coordinate."
Noam Elkies ...

**2**

votes

**0**answers

138 views

### Minimally 6-connected 3D discrete lines that are convex lattice sets

There are several definitions of 3D discrete lines, e.g. http://diwww.epfl.ch/w3lsp/publications/discretegeo/nratddl.html , http://dx.doi.org/10.1007/978-3-642-19867-0_4 . However, I know of none that ...

**3**

votes

**0**answers

131 views

### A fast way to test whether a partial function can be extended to a chirotope of rank 3 ?

Hello,
What is a fast way to test whether a partial function can be extended to a chirotope of rank 3 ? That is: I have a domain
$E = \{1,...,n\}$
and a partial function
$f: E^3 \to \{-1, 0, 1\}$
...

**18**

votes

**3**answers

793 views

### Growing random trees on a lattice $\rightarrow$ Voronoi diagrams

Imagine growing trees from $k$ seeds on a square $n \times n$ region
of $\mathbb{Z}^2$.
At each step, a unit-length edge $e$ between two points of
$\mathbb{Z}^2$ is added.
The edge $e$ is chosen ...

**8**

votes

**3**answers

1k views

### Undecidable problems in geometry

Are there any (many) algorithmically undecidable problems in computational (combinatorial/discrete) geometry?
Update: the Wang tiles answer the question with "any". (I have somewhat overlooked to ...

**4**

votes

**2**answers

475 views

### intersections of real algebraic sets (a bezout-type question)

This question arose when I was trying to understand the Guth-Katz paper on Erdos distance problem, namely, related to Proof of Lemma 2.9 in http://arxiv.org/abs/1011.4105. The proof of that lemma is ...

**3**

votes

**1**answer

267 views

### Enumerating Connected Circle Graphs

Hi
A circle graph is defined as the intersection graph of a set of chords of a circle.
I'm interested in any information which might help to enumerate connected circle graphs.
Thanks
Andy

**2**

votes

**3**answers

818 views

### Draw a Random Line Through a Voronoi Tessellation, What is the Average Number of Voronoi Cell the Line Intersects?

Update: problem reformulation
Following the advice in comments, I now restate my problem using Voronoi
tessellation.
Given a unit hypercube $H_n=\{(x_1,\ldots,x_n)\in \mathbb{R}^n: 0\leq x_i\leq
...

**8**

votes

**3**answers

653 views

### Erdos-Szekeres in high dimensions

All the point sets in this post are in general position. A set of points in $R^d$ is in general position if every $k+1$ points are affinely independent for $k \le d$. If the set contains at least ...

**4**

votes

**1**answer

331 views

### Intersection of boundary facets of a simplicial complex

Suppose you have an equidimensional $n$-dimensional simplicial complex $\Delta \subseteq \mathbb Q^n$; i.e., $\Delta$ is the union of finitely many $n$-simplices that intersect only along proper ...

**6**

votes

**0**answers

256 views

### A question about a blue fan and a red fan and their common refinement

Is the following conjecture true?
Conjecture: Let $M_1$ be a red map and let $M_2$ be a blue map drawn in general position on $S^n$, and let $M$ be their common refinement. There is a vertex $w$ of ...

**6**

votes

**2**answers

264 views

### Small and large pieces of the plane, after countably many generic straight cuttings

A delightful recent problem about disconnecting the plane by straight lines suggested me the following further question, that I can't resist to post.
Let $\mathcal{F} $ be a countable family of ...

**9**

votes

**1**answer

572 views

### infinite configuration of lines

I was looking at some random problems and questions I liked when I was in high school and I found this one which I still cannot prove.
Does there exist a configuration of a countable number of ...

**2**

votes

**1**answer

423 views

### Lipschitz constant of Laplace-Beltrami Operator

I already asked this question at stackexchange with no response - so I'll try here.
I'm reading a paper on discrete differential geometry:
Meyer et.al.
They define the Laplace-Beltrami operator at ...

**1**

vote

**2**answers

293 views

### Can a set of tetrahedra glued together by a common vertex be isometrically embedded in R^4?

A collection of triangles with a common vertex $A_1VA_2$, $A_2VA_3$, ... $A_NVA_1$ with specified side lengths can be isometrically embedded in $R^2$ provided the angles around $V$ add up to $2\pi$. ...

**19**

votes

**1**answer

454 views

### Voronoi cell of lattices with the same profile

Definition 1. Given a body $V$ in $\mathbb R^n$,
the function $p_V\colon \mathbb R_+\to \mathbb R_+$
$$p_V(r)=\mathop{\rm vol} [V\cap B_r(0)]$$
will be called profile of $V$.
Definition 2. Define ...

**12**

votes

**1**answer

649 views

### Random polycube shapes

I am wondering if it is hopeless to obtain any firm results
on the following model of a "random polycube shape."
First, a polycube in $\mathbb{R}^3$
is a connected face-to-face gluing of unit cubes.
...

**13**

votes

**1**answer

529 views

### Which sets of lattice points have rational generating functions?

Let $P$ be a subset of $\mathbb N^d$ (or of some normal pointed affine semigroup), and suppose that $f:=\sum_{p\in P}\ t^p\in\mathbb Z[[t_1,\ldots,t_d]]$ is a rational function. What can be said ...

**3**

votes

**2**answers

500 views

### Torus in $\mathcal{R}^3$

Hi
I'm interested in packing the 3 space as dense as possible using equally sized tori whose major radius is much bigger than their minor radius in.
Do you have any idea how to attack this problem?
...

**20**

votes

**3**answers

1k views

### Rolling-ball game

The analyses
in two recent MO questions,
"Rolling a random walk on a sphere"
and
"Maneuvering with limited moves on $S^2$,"
suggest a Rolling-Ball Game, as follows.
A unit-radius ball sits on a grid ...

**10**

votes

**2**answers

569 views

### Helly theorem + Nerve

Consider nerve $\mathcal N$ of a finite set of convex sets in $\mathbb R^n$.
Helly theorem says that $\mathcal N$ is completely determined by its $n$-skeleton, say $\mathcal N_n$.
It seems that not ...

**6**

votes

**0**answers

208 views

### balls in arrangements of hyperplanes

The following theorem is from Aronov, Naiman, Pach and Sharir's
An invariant property of balls in arrangements of hyperplanes. I would like to state them and then ask if any related problem/theorem ...

**1**

vote

**1**answer

297 views

### Pointers/Papers on subdivision of planar quadrilateral meshes (PQ-Mesh) in 3D?

I'm interested in the subdivision of planar quadrilateral meshes (PQ-Meshes). Meshes consisting only of planar quadrilaterals, like discrete Voss surfaces and alike. I've been searching the web
for ...

**7**

votes

**0**answers

223 views

### Coloring toroidal polyhedra with convex faces?

Consider a toroidal polyhedron, which is a topological torus, in which all faces are planar, two faces meet in at most an edge, and adjacent faces are not coplanar. The Szilassi polyhedron has 7 ...

**7**

votes

**2**answers

757 views

### The straightest possible path embeddable in a path of polygons

I'm studying a problem involving the sets of discrete curves that can be embedded in a non-trivial polygon, from a source to a target point, as shown below.
Initially my interest was limited to ...

**30**

votes

**1**answer

1k views

### Pach's “Animals”: What if genus $> 0$ ?

Janos Pach asked a deep question 23 years ago (1988) that remains unsolved today:
Can every animal—a topological ball in $\mathbb{R^3}$
composed of unit cubes glued face-to-face—be ...

**20**

votes

**2**answers

889 views

### Forbidden mirror sequences

Let $\cal{M}$ be a finite collection of two-sided mirrors,
each an open unit-length segment in $\mathbb{R^2}$,
and such that the segments when closed are disjoint.
A ray of light that reflects off the ...

**55**

votes

**5**answers

2k views

### Does every polyomino tile R^n for some n?

This is a question posed by Adam Chalcraft. I am posting it here because I think it deserves wider circulation, and because maybe someone already knows the answer.
A polyomino is usually defined to ...

**3**

votes

**1**answer

529 views

### Voronoi polygons of general point patterns forced to honeycomb?

This question comes out of a simple wish for graphical displays of people (say members of the Wikimedia chapter for whom I now work) that will display demographic clustering. Obviously in typical ...

**8**

votes

**3**answers

2k views

### n-dimensional voronoi diagram

Hi, I need to compute the voronoi diagram of a set of points in $R^n$.
I'm quite unschooled on the topic, could someone point me to the right references so that I can
a) understand the theory behind ...

**3**

votes

**1**answer

209 views

### Non-inherited symmetries of shadows of point sets

Sometimes a point set in Euclidean space may have a shadow with an unexpected symmetry. The purpose here is to ask when this happens or when it doesn't happen (in some generality).
This requires a ...

**7**

votes

**0**answers

266 views

### 3-dimensional Cayley graph

I would like to see Cayley graphs drawn in 3-dimensional Euclidean space such that the vertices are represented by points and various shadows display the actions of the generators.
For example, ...

**4**

votes

**1**answer

311 views

### Discrete gradient ascent cycles

I am wondering what can be inferred when a discrete
gradient ascent algorithm gets stuck in a cycle.
Here is the situation.
A function $f(x,y)$ is defined over a range $[0,n]^2$,
and the algorithm ...