All Questions
2,368 questions
6
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366
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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
0
answers
99
views
Minimum of the maximum element frequency given the family size and the universe size
[Crossposted at math.stackexchange].
Consider families of sets $\mathcal{F}$ with size $n = |\mathcal{F}|$ and universe $U(\mathcal{F})$ with size $q = |U(\mathcal{F})|$.
I have written and solved ...
- 988
1
vote
1
answer
115
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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
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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
0
answers
68
views
Name of the perspector of the orthic triangle and excentral triangle
The orthic triangle and tangential triangles of a given triangle are in perspective. What's the official kimberling center associated with this perspector?
- 1,109
3
votes
1
answer
285
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Name this kimberling center
The lines which connect the vertices of a triangle with the tangent points between the Spieker circle and the medial triangle are concurrent. Which kimberling center does this point correspond to?
- 1,109
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
3
votes
1
answer
129
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Why is the Vietoris–Rips complex $\operatorname{VR}(S, \epsilon)$ a subset of the Čech complex $\operatorname{Čech}(S, \epsilon\sqrt{2})$?
$\DeclareMathOperator\Cech{Čech}\DeclareMathOperator\VR{VR}$I am reading Fasy, Lecci, Rinaldo, Wasserman, Balakrishnan, and Singh - Confidence sets for persistence diagrams (see here for a version of ...
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
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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
2
votes
0
answers
63
views
Convex planar regions such that every boundary point has a 'fair bisector' passing thru it
We add a little to On 'fair bisectors' of planar convex regions and A claim on the concurrency of area bisectors of planar convex regions .
A fair bisector of a planar convex region is a line ...
- 5,979
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 ...
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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 ...
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1
vote
1
answer
103
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Algorithm to find largest planar section of a convex polyhedral solid
We add a bit more on shadows and planar sections following On a pair of solids with both corresponding maximal planar sections and shadows having equal area . We consider only polyhedrons.
Given a ...
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3
votes
0
answers
136
views
If all max area planar sections of a solid are centrally symmetric, will the solid as whole be centrally symmetric?
It is known that every planar section of an ellipsoid is an ellipse - a centrally symmetric planar figure.
Are there convex solids other than ellipsoids with the property that all its planar sections ...
- 5,979
3
votes
1
answer
253
views
Nagel line of a tetrahedron?
It's well known that there is an analogy for the Euler line in a tetrahedron, but is there also an analogy for the nagel line of a tetrahedron? I can't seem to find any decent literature talking about ...
- 1,109
1
vote
1
answer
134
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An algorithm to arrange max number of copies of a polygon around and touching another polygon
A related post: To place copies of a planar convex region such that number of 'contacts' among them is maximized
Basic question: Given two convex polygonal regions P and Q, to arrange the max ...
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6
votes
1
answer
399
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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
1
vote
0
answers
97
views
Problem related to crossing number
Let $G$ be a graph embedded in the plane (with crossings). For $ F \subset E(G) $, denote by $c(F)$ the set of edges of $G$ that cross some edge in $F$.
Denote $\delta(v)$ the set of edges with one ...
- 111
0
votes
0
answers
115
views
Software for computing polytopes
As can be inferred from the title, I want to do some computation on the facets representation of the polytopes given the vertices. My advisor recommended me Polymake, which is indeed useful even with ...
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1
vote
0
answers
42
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On a pair of solids with both corresponding maximal planar sections and shadows having equal area
This post pulls together Are two convex solids with all corresponding shadows equal in area congruent? and
What can be said about 2 convex solids with corresponding maximal planar sections having ...
- 5,979
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 $C$...
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1
vote
0
answers
59
views
What can be said about 2 convex solids with corresponding maximal planar sections having equal area?
This post follows Are two convex solids with all corresponding shadows equal in area congruent?
Every convex 3D body has planar sections with normals in any given direction. We consider the maximum ...
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2
votes
1
answer
302
views
Are two convex solids with all corresponding shadows equal in area congruent?
By shadow we mean the orthogonal projection of a convex 3D body P onto a 2D plane, for example, the shadow on the xy-plane, with P above (z>0) that plane and the light at L=(0,0,+∞). P an be freely ...
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1
vote
0
answers
79
views
Touring a sequence of convex polygons with minimal energy
There is a known problem of touring a sequence of $n$ polygons: given a starting point $s$, an ending point $t$ and a sequence of polygons $P_1,\dots,P_k$ with a total of $n$ vertices, find points $...
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5
votes
0
answers
145
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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-...
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2
votes
0
answers
81
views
Lattice in a simply connected nilpotent Lie group
Given a connected and simply connected nilpotent Lie group $N$ with a left invariant metric, we assume that there is a lattice $\Gamma$ of $G$. Let $B_1(e)$ be the $1$-ball at the identity element in $...
- 21
2
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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
5
votes
1
answer
176
views
Efficient counting of integer solutions to linear system
In my research, I have a particular 18x18 matrix $\mathbf{A}$ which defines the linear system $\mathbf{A}\cdot \mathbf{x} \leq \mathbf{-1}$ over the nonnegative integers. And I'm interested in ...
- 151
2
votes
0
answers
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
226
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?...
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1
vote
0
answers
93
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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 ...
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0
votes
0
answers
78
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 ...
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0
votes
0
answers
38
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Sequence of projections that alters a $2^d$ tuple of points to a hyperparallelepiped
Suppose we have a $2^d$ tuple $\{ x_i \}_{i=0}^{2^d-1}$ of points in some $\mathbb{R}^n$. I would like to shift the points of this tuple in some controlled way, so that the final $2^d$ tuple $\{ y_i \}...
- 820
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?
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3
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1
answer
108
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Has this random process been studied on grid graphs?
As an offshoot of a different discussion I got curious about (uniform) random spanning trees on grid graphs (torus graphs in particular, to avoid having to think about edge effects) and what their ...
- 1,951
0
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0
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42
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On cutting polyhedrons into convex polyhedral pieces all with same volume, surface area and total edge length
This is a constrained version of the 'fair partition' ('spicy chicken' - https://arxiv.org/abs/1306.2741) question.
It seems that there are convex polyhedrons that cannot be cut into n convex pieces ...
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9
votes
1
answer
542
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Tracking a reference: "Karl Scherer, A Puzzling Journey to the Reptiles and Related Animals"
I linked a paper by James Schmerl in a recent question which cites Karl Scherer, A Puzzling Journey to the Reptiles and Related Animals, Privately Published, 1987.
I have had difficulty finding any ...
1
vote
0
answers
44
views
On area bisectors and perimeter bisectors of planar convex regions
We try to proceed from A claim on the concurrency of area bisectors of planar convex regions
Definitions: Given a planar convex region C, an area bisector of C is any line segment that partitions C ...
- 5,979
15
votes
1
answer
529
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Dividing a polyhedron into two similar copies
The paper Dividing a polygon into two similar polygons proves that there are only three families of polygons that are irrep-2-tiles (can be subdivided into similar copies of the original).
Right ...
0
votes
0
answers
64
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Relation between a cycle on a toroidal graph and divisors of elliptic curve over complex plane
I am very new to algebraic geometry. I was reading about divisors on a scheme. I am wondering does there is some connection between the followings.
An elliptic curve over the complex plane we can ...
- 613
1
vote
0
answers
40
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Why disks might be special - on chords that cut off segments of a specified area from a planar convex body
This post presents a variant of On segments of equal area cut from planar convex regions by chords
Consider a planar convex region C of unit area and all chords of it that cut off a segment of area α ...
- 5,979
3
votes
1
answer
399
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Counting lattice points inside a parallelepiped
The problem I am about to state is in three dimensions and does not follow from Davenport's theorem. Its two-dimensional version is an immediate consequence of Pick's theorem.
Consider the lattice $\...
- 312
1
vote
0
answers
52
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'Self-similar and perfect' partitions of planar regions
Definition: A partition of a planar figure into finitely many pieces that are all similar to itself and also mutually non-congruent may be called a self-similar perfect partition.
A classical example ...
- 5,979
10
votes
2
answers
254
views
Is the face lattice of the cube a polytope graph?
The face lattice of a
convex polytope $P\subset\Bbb R^d$ is the partially ordered set whose elements are the faces of $P$ ordered by inclusion. We can turn it into a graph by considering its Hasse ...
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