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

6 votes
1 answer
176 views

Seeking a Weyl tube formula for Whitney stratified spaces

Background: Let $X$ be a smooth, compact Riemannian submanifold of euclidean space $\mathbb{R}^n$. H Weyl's tube formula asserts that for sufficiently small $t > 0$, the volume $V(X;t)$ of the radius- …
12 votes
4 answers
1k views

For which metric spaces is Gromov-Hausdorff distance actually achieved?

Question For which pairs $M,N$ of compact metric spaces does there exist a metric space $K$ along with isometric embeddings $i:M \to K$ and $j:N \to K$ so that the Hausdorff distance between $i(M) …
5 votes
3 answers
1k views

What fraction of a sphere's volume lies within a cone?

Let $B \subset \mathbb{R}^n$ be a collection of $n$ (not necessarily independent) unit vectors which we will label $v_1,\ldots, v_n$ for convenience. The cone $K_B \subset \mathbb{R}^n$ associated to …
14 votes
5 answers
581 views

When does a pair of homotopic Lipschitz functions fail to admit a Lipschitz homotopy?

Let $(M,d)$ and $(N,\rho)$ be metric spaces. A function $f: M \to N$ is Lipschitz if there exists some constant $\kappa \geq 0$ so that $\rho(f(x),f(y))$ is smaller than $\kappa d(x,y)$ for all points …
4 votes
0 answers
95 views

Name for metric spaces with useful unique-ball-intersection property?

When dealing with the problem of extending a Lipschitz function $f:A \to Y$ between metric spaces across an inclusion $A \hookrightarrow X$, one often imposes (conditions which imply) the following pr …
8 votes

Do cotangent bundles have "bounded geometry"?

Let me complement (and compliment?) Igor's answer by providing an explicit definition of bounded geometry from the work of Cheeger and Gromov. A Riemannian manifold $(M,g)$ has $C^k$-bounded geometr …
Vidit Nanda's user avatar
  • 15.5k
3 votes
0 answers
57 views

Algorithm to construct metric space endomorphism with controlled square

Given a finite metric space $(M,d)$ with parameters $K \geq 1$ and $\epsilon > 0$, I'd like to algorithmically check for the existence of a non-identity map $\phi:M \to M$ which happens to be $K$-biLi …
8 votes
1 answer
412 views

Areas of Triangles in (Non-Riemannian) Metric spaces?

I'm looking for a reasonable way to coherently axiomatize both length and area in the absence of a Riemannian structure, i.e., starting only with a metric space; but it's not clear how much of this wo …
5 votes
2 answers
561 views

Covering convex polygons with inscribed disks

The following problem came up when discussing mapping software (e.g., Google maps) with computer scientists. By $B(c,r)$ I mean the planar disk (open or closed, it doesn't matter) of radius $r$ around …
26 votes
Accepted

Metrics for lines in $\mathbb{R}^3$?

There is no hope of making any such metric continuous with respect to $\theta$ at $s=0$ without violating the triangle inequality even in the plane. Consider parallel lines $L_1$ and $L_2$ which are s …
Vidit Nanda's user avatar
  • 15.5k
1 vote
Accepted

Sets invariant under sections

Your setup defines set-valued dynamics on $X$. More precisely, you have $$X \stackrel{p}{\leftarrow} \overline{G} \stackrel{q}{\rightarrow} X$$ where $p$ and $q$ are the obvious projection maps. The s …
Vidit Nanda's user avatar
  • 15.5k
4 votes

Metrically homogeneous subsets of the plane

There's a complete classification that has been known since 1967: look past the Springer paywall here in the paper Branko Grünbaum, L. M. Kelly. Metrically homogeneous sets, Israel Journal of Mathema …
Vidit Nanda's user avatar
  • 15.5k
9 votes

What is a good method to find random points on the n-sphere when n is large?

Nice question! I ran into a similar problem a few years ago -- even for dimension $10$, the rejection method was annoyingly slow. One of the problems is that such questions straddle at least three hug …
Vidit Nanda's user avatar
  • 15.5k
13 votes
3 answers
832 views

What fraction of n-point sets in the unit ball have diameter smaller than 1?

This question is inspired by a recent talk by Matt Kahle on random geometric complexes. Some simple notation: let $\mathcal{B} \subset \mathbb{R}^d$ be the unit ball in $d$-dimensional Euclidean spa …
2 votes
1 answer
125 views

Interpolating Bijections of Point Sets in Euclidean Space

Consider two finite point sets $P$ and $Q$ from (the same) Euclidean space, assume that they have the same cardinality $n$ and fix a bijection $\phi:P\to Q$. Define an undirected bipartite graph $G_\p …