All Questions
Tagged with mg.metric-geometry discrete-geometry
671 questions
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Asymptotic optimal sphericity
How quickly does maximum sphericity of polyhedra with $n$ faces approach 1 as $n→∞$? I can show that sphericity $1 - \frac{5 \sqrt{3} π}{27n} - O(n^{-3/2})$ is possible. Is this, especially $O(n^{-3/...
- 7,545
1
vote
1
answer
144
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On convex polygons contained in convex polygons
In what follows '$n$-gon' stands for '$n$-vertex polygonal region'.
Question: Given a convex $n$-gon $C$, find the smallest convex region $R$ such that $C$ is the smallest $n$-gon that contains it.
...
- 5,979
2
votes
0
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131
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Cutting polygons into mutually similar and non-congruent pieces
It is well-known that a square can be cut into a finite number of squares all of mutually different sides (hence mutually non-congruent) - for example, see https://en.wikipedia.org/wiki/...
- 5,979
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48
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Deployment and dispersion in triangular regions
Definitions (from C. Stanley Ogilvy's 'Tomorrow's Math'):
Deployment: To place a specified number $n$ of points (stations) in a region such that the maximum distance of any point in the region from ...
- 5,979
3
votes
0
answers
120
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On $n$-gons Inscribed in convex closed curves
Given an integer $n$ and a convex closed planar curve $C$ ($C$ could be smooth). We need to put $n$ points on $C$ such that (1) the area of the convex hull of these points is maximum. (2) perimeter of ...
- 5,979
31
votes
5
answers
1k
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Fair cutting of the plane with lines
An infinite countable family $\cal{L}$ of straight lines in the plane $\mathbb{R}^2$ forms a fair cutting of the plane if the following conditions are satisfied:
$\bullet$ No circle intersects ...
- 7,276
2
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0
answers
131
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Maximum number of regions in a disk partitioned by pairs of parallel chords
We are given a disk $D$ in $\mathbb{R}^2$. Let $C$ be its boundary (i.e., the circle bounding $D$ on its plane). Let $P(n,d)$ be a set of $n$ pairs of chords of $C$ such that for each $\{c,c'\}\in P(n,...
- 1,677
5
votes
2
answers
366
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Infinitely long rods that touch one another
Background: The basic question as given in 'Research Problems in Discrete Geometry' By Moser, Brass and Pach (page 98) is: What is the max number of congruent infinite circular cylinders that can be ...
- 5,979
8
votes
1
answer
508
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'Fattest' polygons based on diameter and 'least width'
Definitions: The diameter of a convex region is the greatest distance between any pair of points in the region. The least width of a $2$D convex region can be defined as the least distance between any ...
- 5,979
5
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190
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The existence of $n$-sided cells in regular $m$-gons
For any integer $n >= 3$, does there exist a regular
$m$-gon with all diagonals drawn containing a cell with $n$ sides?
See A342222 and its cross-references.
Regular polygon on the Wiki.
&...
- 251
2
votes
1
answer
151
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Given an input point in $\mathbb{R}^n$, select (one of) the closest point(s) from a fixed large set of points given in advance
We are given a set $S$ of $m\gg 1$ points in $\mathbb{R}^n$.
In the problem I am trying to solve, in a sequential fashion, we obtain a new point $p_r\not\in S$ at each round $r\ge 1$ and the goal is ...
- 1,677
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0
answers
90
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On Covering a Planar Region with Copies of a Tile of Different Shape
Background: Consider trying to cover the largest possible scaled copy of a planar region $C$ with specified shape with n instances of a tile $T$ of specified shape and size. Several families of this ...
- 5,979
4
votes
1
answer
215
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On two centers of convex regions
Definition: A line segment with both end points on the boundary of a planar convex region $C$ is called a chord of $C$.
Consider any point $P$ within a given planar convex region $C$. From among all ...
- 5,979
2
votes
1
answer
143
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Triangles and convex hulls in high dimensions
Given a set $S_n$ of $n$ points $\mathbf{x}_1, \mathbf{x}_2, \ldots, \mathbf{x}_n\in\mathbb{R}^d$, such that every $(d+1)$-tuple in $S_n$ is affinely independent, and let $C(S_n)$ be the convex hull ...
- 1,677
6
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112
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Which $n$-gons of diameter 1 maximize the moment of inertia?
Background: Among convex plane $n$-gons of unit diameter, we can try to achieve:
the largest area. (This is called the biggest little polygon with $n$ sides; for $n$ odd, the regular polygon on $n$ ...
- 5,979
3
votes
0
answers
92
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To what extent can I specify the angular defect at the vertices of a polyhedron?
Suppose $p_1,\ldots,p_n$ are points in $\mathbb{R}^3$, and suppose $\delta_1,\ldots,\delta_n$ are positive real numbers, each less than $2\pi$, whose sum is $4\pi$. Is there a polyhedron $\mathcal P$ ...
- 326
2
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1
answer
202
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To cut a triangle into $n$ $p$-sided polygonal regions
Given any triangular region and two integers $n$ and $p$ which can be large and $p > 4$. It is needed to cut the triangle into $n$ $p$-gons (e.g., cut a triangle into 10 heptagons). Among the $p$-...
- 5,979
3
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40
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Are there uniform compounds of 135 $BC_8$ polytopes?
The Coxeter group $D_8$ is an index-135 subgroup of $E_8$. One of the consequences of this is that the rectified 8-orthoplex, whose coordinates can be given as all permutations and sign changes of $\{...
- 2,773
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0
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124
views
Number of lattice points in a structural symmetric convex body
Let $f$ is a convex symmetric function on the interval $[-a,a]$, i.e., $f(-x)=f(x)$ for $\forall \, x\in [-a,a]$. Then we consider a $n$-dimensional convex body in Euclidean space
\begin{equation}
\...
- 550
3
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103
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Are there any other regular compounds?
Ever since I first read Coxeter’s definition of a regular compound (which seems to be the most commonly used), I didn’t like it on account of it being completely different than for properly connected ...
- 2,773
6
votes
2
answers
544
views
On circles and ellipses drawn on an infinite planar square lattice
Consider a plane with a square lattice formed by all points with both coordinates as integers. As can be easily seen, a simple parabola can be found that passes through infinitely many of the square ...
- 5,979
2
votes
1
answer
151
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On congruent partitions of planar regions
Given any integer $n$, any rectangular region or any sector of a disc (including the full disk as a boundary case) can be cut into $n$ mutually congruent pieces - by equally spaced parallel lines and ...
- 5,979
22
votes
1
answer
886
views
Happy ants never leave compact domain?
I am curious if the following seemingly simple question has an easy answer?
Consider an ant population of $N$ ants that lives in $\mathbb R^2$. Each ant can be labeled by some coordinate $x\in \mathbb ...
- 124
3
votes
1
answer
143
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Combinatorial Euclidean geometry problem
Let $\mathcal{S}^d_{\epsilon}$ be the collection of all sets $S:=\{\mathbf{x}_1, \mathbf{x}_2, \ldots \mathbf{x}_{d+1}\}$ of $d+1$ points in a $d$-dimensional Euclidean space such that, for a given ...
- 1,677
4
votes
0
answers
232
views
Illuminating a just-barely irrational polygon
As has been discussed earlier on MO,1,2
recently an impressive advance was proved concerning
internally illuminating a mirrored polygon.
Here is the result:
Let $P$ be a rational polygon.
Then for ...
- 151k
8
votes
0
answers
249
views
Approximating a general rectangle partition by a guillotine partition
There is a rectangle $R$ partitioned into some axes-parallel rectangles:
The goal is to construct another partition of $R$ into rectangles, using only guillotine cuts.
That is: cut $R$ into two ...
- 3,549
1
vote
1
answer
89
views
Vertices of 2 self-polar triangles lie on conic
I have conic $\gamma$ and two self-polar triangles $ABC$, $XYZ$ with respect to my conic. Why can I construct a one conic through $ABCXYZ$?
- 49
5
votes
1
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264
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Is there a polytope with an essentially unique shape?
More percisely:
Question: Is there a (convex) polytope that has a unique realization up to, say, projective transformations?
I suppose I have to assume that it has more than $d+2$ vertices/facets if ...
- 13.6k
3
votes
2
answers
438
views
If a polytope is centrally symmetric and combinatorially equivalent to a zonotope, is it a zonotope?
A zonotope is a polytope whose 2-faces are centrally symmetric.
Question: If a polytope $P$ is centrally symmetric and combinatorially equivalent to a zonotope, is it itself a zonotope?
- 13.6k
4
votes
2
answers
294
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Is there more than one pseudo-Catalan solid?
This question was asked on MSE a year ago. Motivation for this question can be found in other MSE questions here, here or here.
Convex solids can have all sorts of symmetries:
the platonic solids are ...
- 4,432
2
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1
answer
404
views
Euclidean distance bound with geometric constraints
Let $S_n$ be a set of $n$ points belonging to $\mathcal{B}_d:=\{\mathbf{x}\in\mathbb{R}^d:\|\mathbf{x}\|_2\le 1\}$, where $d\ll \log(n)$.
Let $s_n$ and $\ell_n$ be respectively defined as follows:
$$...
- 1,677
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0
answers
93
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A variant of the Mondrian problem
Definition: The Mondrian problem consists of dissecting a square of side length n (an integer) into mutually non-congruent rectangles with integer length sides such that the difference d(n) between ...
- 5,979
4
votes
2
answers
341
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Cutting convex regions into equal diameter and equal least width pieces - 2
This post is a spinoff from Cutting convex regions into equal diameter and equal least width pieces
Definitions: The diameter of a convex region is the greatest distance between any pair of points in ...
- 5,979
9
votes
1
answer
160
views
Hyperplane arrangements whose regions all have the same shape
Suppose I have a (finite, real, central, essential) hyperplane arrangement $\mathcal{H}$ such that all regions "have the same shape": for any two regions $R,R'$, there is an orthogonal ...
- 2,753
13
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0
answers
573
views
What are the known convex polyhedra with congruent faces?
Note: I originally asked this question on math.SE here, where I posted a bounty on the question but received no answers after a week despite apparent interest in the problem. I'm hoping MathOverflow ...
- 3,068
2
votes
1
answer
209
views
Cutting convex regions into equal diameter and equal least width pieces
The diameter of a convex region is the greatest distance between any pair of points in the region.
The least width of a 2D convex region can be defined as the least distance between any pair of ...
- 5,979
3
votes
1
answer
381
views
Source on counting lattice points on a line
Looking for a book or article on the result linked below. The result tells us that the number of lattice points on a line between points $(a,b)$ and $(c,d)$ is given by $\gcd(a-c,b-d)+1$.
https://math....
- 45
2
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0
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131
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Optimal way to group points in the plane into clusters
Consider a strictly decreasing sequence $d = (d_k)_{k\ge 1}$ of distances in $(0,1)$. Given a constant $C>2$, we say that $d$ has the $C$-grouping property if any finite non-empty subset $S$ (of ...
- 1,761
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0
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81
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Constructive way to optimally cover a compact subset of Euclidean space
Let, $(X,d)$ be a simply connected compact subset of $\mathbb{R}^d$ with non-empty interiorn, let $d$ denote the Euclidean metric, and let $\varepsilon>0$. Is there a way to iteratively select ...
- 5,405
3
votes
1
answer
190
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On some centers of convex regions based on partitions
These questions are inspired by Yaglom and Boltyanskii's 'Convex figures'.
Winternitz Theorem: If a 2D convex figure is divided into 2 parts by a line $l$ that passes through its center of gravity, ...
- 5,979
11
votes
1
answer
652
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How to correctly state Cauchy's rigidity theorem?
Cauchy's rigidity theorem is often stated briefly as
Any two (convex, 3-dimensional) polyhedra with pairwise congruent faces are themselves congruent.
As a more formal generalization to general ...
- 13.6k
5
votes
0
answers
93
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Which polytopes can be deformed while keeping their edge-lengths?
Let $P\subset\Bbb R^d$ be a convex polytope (a convex hull of finitely many points). Lets call it flexible, if it can be continuously deformed while
keeping its combinatorial type, and
keeping its ...
- 13.6k
5
votes
2
answers
134
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Is there a non-orthogonal linear deformation of a polytope that preserves edge-lengths and vertex-origin-distances?
Is there a polytope $P\subset\Bbb R^d$ (convex hull of finitely many points, not contained in a proper affine subspace), and a linear, but non-orthogonal transformation $T\in\mathrm{GL}(\Bbb R^d)\...
- 13.6k
19
votes
1
answer
928
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Can every simple polytope be inscribed in a sphere?
It is known that not every convex polytope (even polyhedron, e.g. this one) can be made inscribed, that is, we cannot always move its vertices so that
all vertices end up on a common sphere, and
the ...
- 13.6k
5
votes
1
answer
114
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Packing in uniform domains
Given $N$ points $X:=(x_i)_{i \in \{1,..,N\}}$, we now define a score function $S:X \rightarrow \mathbb{N}$ that is $S(X)= \sum_{i=1}^N S(x_i)$ where the score of $S(x_i)$ is
$$S(x_i) = 2* \vert \{x_j;...
- 536
4
votes
3
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347
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Minimal data required to determine a convex polytope
Let $P\subset \Bbb R^d$ be a convex polytope.
Suppose that I know
its combinatorial type (aka. the face-lattice),
the length $\ell_i$ of each edge, and
the distance $r_i$ of each vertex from the ...
- 13.6k
5
votes
0
answers
313
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Trade-off between covering number, ball radius and diameter of $d$-dimensional shapes
Given any $d$-dimensional shape $X$ in the Euclidean space, let $\ell(X)$ be the length of the longest line segment connecting two points of $X$. How can we prove the following statement?
There exists ...
- 1,677
3
votes
0
answers
134
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Two questions on counterexamples to Borsuk's conjecture and ball-packings
In 1933 Karol Borsuk conjectured the following
Can every bounded subset $E$ of $\mathbb{R}^d$ be partitioned into $(d+1)$ sets, each of which has a smaller diameter than $E$?
Whilst new to this ...
- 31
6
votes
1
answer
244
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Is Sydler's theorem concerning Dehn invariants constructive?
Sydler proved something of a converse to Dehn's negative resolution
of Hilbert's 3rd problem. To quote Wikipedia, Sydler showed that
"every two Euclidean polyhedra with the same volumes and Dehn ...
- 151k
12
votes
1
answer
373
views
A claim on partitioning a convex planar region into congruent pieces
Let us define a perfect congruent partition of a planar region $R$ as a partition of it with no portion left over into some finite number n of pieces that are all mutually congruent (ie any piece can ...
- 5,979