All Questions
4,827 questions
22
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1
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Characterization of Volumes of Lattice Cubes
This is a cross post from Math SE that no one seemed able to solve.
Here is a problem that came up in a conversation with a professor after I made a false assumption about the geometry of $\mathbb{Z}^...
- 612
22
votes
2
answers
1k
views
Why doesn't this construction of the tangent space work for non-Riemannian metric manifolds?
In the 1957 paper, On the differentiability of isometries, Richard S. Palais gives a way to construct the tangent spaces of a Riemannian manifold using only its metric space structure (Theorem, p.1).
...
- 2,680
22
votes
1
answer
1k
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Just how close can two manifolds be in the Gromov-Hausdorff distance?
Suppose that we have two compact Riemannian manifolds $(M,g)$ and $(N,h)$. Define the Gromov-Hausdorff distance between them in your favorite way, I'll use the infimum of all $\epsilon$ such that ...
- 646
22
votes
1
answer
663
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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 ...
- 45k
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
22
votes
1
answer
1k
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Random distance matrices
My question is motivated by the following recent paper:
Gadgil, Siddhartha; Krishnapur, Manjunath, Lipschitz correspondence between metric measure spaces and random distance matrices, Int. Math. Res. ...
- 3,626
22
votes
0
answers
402
views
What is the covering density of a very thin annulus? Is it $\frac{\pi\sqrt{51\sqrt{17}-107}}{16}$?
Take some very small $\epsilon>0$, and consider the annulus/ring given by the set $\{(r,\theta)\ |\ 1-\epsilon\le r\le1\}\subset \mathbb{R}^2$.
We wish to place translated copies of this annulus ...
- 3,068
21
votes
6
answers
3k
views
Are there smooth bodies of constant width?
The standard Reuleaux triangle is not smooth, but the three
points of tangential discontinuity can be smoothed as
in the figure below (left), from the Wikipedia article.
However, it is unclear (to me) ...
- 151k
21
votes
8
answers
4k
views
Determine if circle is covered by some set of other circles
Suppose you have a set of circles $\mathcal{C} = \{ C_1, \ldots, C_n \}$ each with a fixed radius $r$ but varying centre coordinates. Next, you are given a new circle $C_{n+1}$ with the same radius $r$...
- 311
21
votes
5
answers
1k
views
Is there a midsphere theorem for 4-polytopes?
The (remarkable) midsphere theorem says that each combinatorial
type of convex polyhedron may be realized by one all of whose edges are
tangent to a sphere
(and the realization is unique if the center ...
- 151k
21
votes
5
answers
1k
views
Is a rhombus rigid on a sphere or torus? And generalizations
If a rectangle is formed from rigid bars for edges and joints
at vertices, then it is flexible in the plane: it can flex
to a parallelogram.
On any smooth surface with a metric, one can define a ...
- 151k
21
votes
3
answers
935
views
Cutting of a regular polygon into congruent pieces
Question. For which $N$ it is possible to cut a regular $N$-gon into congruent pieces such that the center of the regular polygon lies strictly inside one of the pieces? For $N=3,4$ there are trivial ...
- 691
21
votes
1
answer
975
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Conjecture: Given any five points, we can always draw a pair of non-intersecting circles whose diameter endpoints are four of those points
The following question resisted attacks at Math SE, so I thought I would try posting it here.
Is the following conjecture true or false:
Given any five coplanar points, we can always draw at least ...
- 3,527
21
votes
2
answers
3k
views
A measure on the space of probability measures
This question was originaly posted in the stackexchange https://math.stackexchange.com/questions/1226701/a-measure-on-the-space-of-probability-measures but since it only got a comment I decided to ...
- 361
21
votes
2
answers
688
views
Gluing hexagons to get a locally CAT(0) space
I believe that there are four ways to glue (all) the edges of a regular Euclidean hexagon to get a locally CAT(0) space:
The first two give the torus and the Klein bottle, respectively. What are the ...
- 10.1k
21
votes
2
answers
1k
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 ...
- 151k
21
votes
1
answer
3k
views
A circle packing conjecture
Consider $n$ circles with variable radii $r_1,\ldots, r_n$ that pack inside a fixed circle of unit radius. In other words, all $n$ variable-radius circles are contained in the unit radius circle and ...
- 1,085
21
votes
1
answer
1k
views
Sphere packings : what next after the recent breakthrough of Viazovska (et al.)?
Given the march 2016 breakthrough concerning sphere packings by Viazovska for the case of dimension 8, and by Cohn, Kumar, Miller, Radchenko and Viazovska for the case of dimension 24, it follows that ...
- 883
21
votes
1
answer
690
views
Diameter of a quotient of the infinite dimensional sphere
Suppose a group $\Gamma$ acts by isometries on the Hilbert space $\mathbb{H}^\infty$ and it fixes the origin. So $\Gamma$ acts on the unit sphere $\mathbb{S}^\infty$ as well.
Assume that the action $...
- 45k
21
votes
1
answer
2k
views
Coiling Rope in a Box
What is the longest rope length L of radius r that can fit into a box?
The rope is a smooth curve with a tubular
neighborhood of radius r, such that the rope does not
self-penetrate. For an open ...
- 151k
21
votes
1
answer
1k
views
Which convex bodies roll along closed geodesics?
An ellipsoid could be rolled (without slippage) on a horizontal plane so that its point
of contact traces out a closed geodesic on its surface:
...
- 151k
21
votes
2
answers
1k
views
Geometric interpretation of exceptional symmetric spaces
Elie Cartan has classified all compact symmetric spaces admitting a compact simple Lie group as their group of motion.There are 7 infinite series and 12 exceptional cases. The exceptional cases are ...
- 3,022
21
votes
2
answers
1k
views
On convergence of convex bodies
Let $K\subset \mathbb{R}^n$ be a compact convex set of full dimension. Assume that $0\in \partial K$.
Question 1. Is it true that there exists $\varepsilon_0>0$ such that for any $0<\...
- 21.8k
21
votes
6
answers
3k
views
Smooth functions on sphere
Let $u$ be a smooth function defined on the unit sphere $S^2$. Assume $u$ has two local maxima, two local minima, and two saddle points (a total of 6 critical points). Does there exist a plane $P$ ...
21
votes
2
answers
1k
views
Probability that a convex shape contains the unit ball
This probability problem seems interesting and I don't know if it has been solved before.
If you pick $n$ points uniformly at random from the surface of a $d$ dimensional sphere of radius $r>1$ ...
- 3,377
21
votes
1
answer
1k
views
A Weak Form of Borsuk's Conjecture
Problem: Let P be a d-dimensional polytope with n facets. Is it always true that P can be covered by n sets of smaller diameter?
Background and motivation
The Borsuk conjecture (disproved in 1993) ...
- 24.7k
21
votes
0
answers
271
views
The "stained glass window problem": Draw many random chords in a circle; which kind of polygon ($3$-gon, $4$-gon, etc.) occupies the most total area?
Draw $n$ random chords in a circle, where each chord connects two independent uniformly random points on the circle.
As $n\to\infty$, which kind of polygon (triangle, quadrilateral, pentagon, etc.) ...
- 3,527
20
votes
8
answers
5k
views
Mathematical theory of aesthetics
The notion of beauty has historically led many mathematicians to fruitful work. Yet, I have yet to find a mathematical text which has attempted to elucidate what exactly makes certain geometric ...
20
votes
2
answers
3k
views
Can a 2-sphere be squashed flat?
Does there exist a function $f:\Bbb{S}^2\rightarrow\Bbb{R}^2$ which preserves the length of every rectifiable curve? That is, can a sphere be crushed flat without tears? Of course, this is a Nash-...
- 430
20
votes
2
answers
25k
views
Partitioning a polygon into convex parts
I'm looking for an algorithm to partition any simple closed polygon into convex sub-polygons--preferably as few as possible.
I know almost nothing about this subject, so I've been searching on Google ...
- 201
20
votes
5
answers
1k
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Historical use of figures in geometry
I was surprised to learn from John Stillwell's comment in answer to the
question,
"Can the unsolvability of quintics be seen in the geometry of the icosahedron?",
that
There is not a single picture ...
- 151k
20
votes
5
answers
1k
views
Probability that biggest area stays greater than 1/2 in a unit square cut by random lines
The square $[0,1]^2$ is cut into some number of regions by $n$ random lines. We can chose these random lines by randomly picking a point on one of the four sides, picking another point randomly from ...
20
votes
7
answers
1k
views
Is there always a maximum anti-rectangle with a corner square?
Let $C$ be an axis-parallel orthogonal polygon with a finite number of sides. Define an anti-rectangle in $C$ as a set of small squares in $C$, such that no two of them are covered by a single large ...
- 3,549
20
votes
2
answers
2k
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"a shape that ... lies halfway between a square and a circle"
An article in the
Notices of the AMS, Volume 61, Issue 10, 2014
(PDF download link),
on Khot's Unique Games Conjecture, says this:
Another group ... found a
shape that in a certain sense lies ...
- 151k
20
votes
3
answers
1k
views
Gromov-Hausdorff distance between p-adic integers.
What is the distance in the sense of Gromov-Hausdorff between $\mathbb{Z}_{p_1}$ and $\mathbb{Z}_{p_2}$ with the usual p-adic metrics?
I got stuck and simply have no idea how to deal with such ...
- 2,934
20
votes
4
answers
950
views
The limit of edge-midpoint convex polyhedra
Starting with a convex polyhedron $P_1 \subset \mathbb{R}^3$,
replace that with $P_2$, the convex hull of the midpoints of the edges of $P_1$.
Continuing this process, we obtain a ...
- 151k
20
votes
5
answers
1k
views
From convex polytopes to toric varieties: the constructions of Davis and Januszkiewicz
One of the most useful tools in the study of convex polytopes is to move from polytopes (through their fans) to toric varieties and see how properties of the associated toric variety reflects back on ...
- 24.7k
20
votes
3
answers
1k
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How can I randomly draw an ensemble of unit vectors that sum to zero?
Inspired by this question, I would like to determine the probability that a random knot of 6 unit sticks is a trefoil. This naturally leads to the following question:
Is there a way to sample ...
- 7,633
20
votes
4
answers
2k
views
Minkowski sum of small connected sets
Suppose that the convex hull of the Minkowski sum of several compact connected sets in $\mathbb R^d$ contains the unit ball centered at the origin and the diameter of each set is less than $\delta$. ...
- 61.9k
20
votes
2
answers
2k
views
The geometric median of a triangle
Let $\Omega\subset \mathbb R^n$ be a compact domain of dimension $n$. Define the geometric median on $\Omega$ as the point $m_{\Omega}\in \mathbb R^n$ such that the integral $\int_{\Omega}|x-m_{\Omega}...
- 14.3k
20
votes
5
answers
4k
views
Advanced view of the napkin ring problem?
The "napkin-ring problem" sometimes shows up in 2nd-year calculus courses, but it can fit quite neatly into a high-school geometry course via Cavalieri's principle.
However, the conclusion remains ...
20
votes
2
answers
1k
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Center of mass from the abstract point of view, or could the ancient Greeks invent modern analysis?
This is a very open-ended question, which may or may not have a perfect answer, and for which I have a few ideas but nothing like a clear picture. However, I guess it won't hurt to ask to see if ...
- 61.9k
20
votes
3
answers
3k
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How many unit squares can you pack into a rectangle with nearly integer side lengths?
Earlier today, somebody asked what looks like a homework problem, but admits the following reading which I think is interesting:
Suppose $a_1,\dots, a_n$ are positive integers, and $\varepsilon$ is ...
20
votes
3
answers
2k
views
Duality between topology and bornology
I want to understand in what sense topology is dual to bornology at a most basic level. Therefore, I rephrased the definition of a bornology in the following way:
Let $X$ be a set and let $\mathcal{P}(...
- 1,813
20
votes
5
answers
3k
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Finding Constant Curvature Metrics on Surfaces without full power of Uniformization
(I rewrote this question, hopefully it's more clear now. It's still the same question, but I reordered its parts.)
Let S be a surface (possibly non-compact, but no boundary). It seems that there are ...
- 3,203
20
votes
1
answer
902
views
Minimal pizza cutting
Given a circle, we want to divide it into $n$ connected equally sized pieces. In such a way that the total length of the cutting is minimal. What can we say about the solution for each $n$. Are they ...
20
votes
1
answer
452
views
Hidden points in polygons
Let $h(n)$ be the largest number of mutually invisible points that can be located in a
polygon $P$ of $n$ vertices. Two points $x$ and $y$ are mutually invisible if the segment
$xy$ contains a point ...
- 151k
20
votes
1
answer
612
views
Is Monsky's theorem provable in $\mathsf{RCA}_0$?
Monsky's theorem states that it is not possible to dissect a square into an odd number of triangles of equal area. Monsky's proof attracted attention in part because it unexpectedly made use of the ...
- 82.7k
20
votes
1
answer
920
views
Infinite desert with waterpoints
UPDATE: I created a simple web-application, that allows the user to move waterpoints around, then automatically calculates a maximum set of interior-disjoint squares between the points (the algorithm ...
- 3,549
20
votes
3
answers
690
views
Escaping from infinitely many pursuers
The fugitive is at the origin. They move at a speed of $1$. There's a guard at $(i,j)$ for all $i,j\in \mathbb{Z}$ except the origin. A guard's speed is $\frac{1}{100}$. The fugitive and the guards ...
- 2,619