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Euclidean, hyperbolic, discrete, convex, coarse geometry, metric spaces, comparisons in Riemannian geometry, symmetric spaces.

3 votes

Complexity of computing kissing numbers of triangles with given side lengths

Too long for a comment: I think there is a reason, why this is algorithmically solvable. Given three side lengths $a,b$ and $c$, and a fixed number $k\in\mathbb{N}$, writing down all the (in)equali …
Moritz Firsching's user avatar
10 votes

A cube is placed inside another cube

The answer to your question is: no Its also easy to give a concrete counter-example: Take the cube with vertices $$ \left(\frac{273}{340},\,\frac{79}{68},\,\frac{13}{20}\right) , \left(\frac{407 …
Moritz Firsching's user avatar
3 votes

name for a polytope constructed from a system of linear equations?

Maybe a term you are looking for is "sparse solutions of underdetermined linear systems". At least this would maybe be a good name for your set $V$. It can be viewed as solution set of the following o …
Moritz Firsching's user avatar
1 vote

Probability that a convex shape contains the unit ball

I hope you can find a partial answer here. At least for $n=d+2$.
Moritz Firsching's user avatar
24 votes

How to check if a box fits in a box?

A (trivial) necessary condition is that the diagonal of the inner one is not longer than the diagonal of the outer one. So if $(a,b,c)$ is supposed to fit in $(x,y,z)$, then we should have $$a^2+b^2+ …
Moritz Firsching's user avatar
15 votes
Accepted

Does Gromov's Waist Inequality imply Borsuk-Ulam?

Yashar Mermarian writes here that the answer is yes. And the argument he gives is pretty much the same as the one you already started. Taking $n=k$ and $\epsilon=\pi/2$ Gromov's waist inequality give …
Moritz Firsching's user avatar
3 votes

Problem equivalent to "largest square in a cube"

When studying this problem in the general case, i.e. $m$-dimensional cube inside $n$-dimensional cube for $m<n$, it might make sense to look at small examples to gain some intuition. From the small ex …
Moritz Firsching's user avatar
7 votes

When does there exist a convex polyhedron with given edge lengths?

Since I expect the answer for all combinatorial types of polytopes to be somewhat obscure, let me give an answer to the more simple question of what happens for the tetrahedron. We denote the six edge …
Moritz Firsching's user avatar
3 votes
Accepted

Hausdorff dimension of Apollonian circle packing, 1.305686729, 1.305688 or yet something else?

It seems in the meantime, there are new results: Bai, Zai-Qiao; Finch, Steven R. Precise calculation of Hausdorff dimension of Apollonian gasket. Fractals 26 (2018), no. 4, 9 pp. claims a better appro …
Moritz Firsching's user avatar
35 votes
Accepted

what-if.xkcd.com: stabbing (simply connected) regions on the 2-sphere with few geodesics

Looking at this old question again, I'm now fairly convinced that the easiest route of solving this problem is using similar ideas to the one suggested by David E Speyer in a comment, namely basically …
Moritz Firsching's user avatar
47 votes
Accepted

How many unit cylinders can touch a unit ball?

Here is an idea. Consider the following parameterization, which is supposed to cover the configuration space in question. $$\mathcal{C}_7:=\left\{\pmatrix{x_k\\y_x\\z_k},\pmatrix{a_k\\b_k\\c_k}_{1\le …
Moritz Firsching's user avatar
10 votes

Largest regular $k$-simplex inscribed in a $d$-cube, $k < d$

Allow me look at one aspect, or special case, of your question, namely "finding the largest regular 3-dimensional tetrahedron inscribed in a d-dimensional unit cube". I. $4$-cube I can find the follow …
Moritz Firsching's user avatar
2 votes

Maximal volume of a simplex inscribed in a spherical cap

I don't know any reference for this, and I don't know if this should be a "classical result", but let me give a lower bound, which might even be tight. Let's denote the base of the cap by $BS$. It is …
Moritz Firsching's user avatar
3 votes

Definition of "regular" in Stringham's "Regular figures in n-dimensional space"

Too long for a comment: Stringham gave a talk about the content of his thesis here in the Seminar of Felix Klein in Göttingen on Monday, 1880/11/29, you can look at the scans here: Ueber reguläre Körp …
Moritz Firsching's user avatar
10 votes

Double kissing problem

Using global nonlinear optimization one can obtain a configuration of $19$ spheres, that touch at least one of the central unit spheres and have almost no overlap. In fact, if one takes their radii to …
Moritz Firsching's user avatar

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