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

2 votes

Volume of a region given by a Constraint Satisfaction Problem

It all depends on how small $n$ is compared to $m,$ but in general, this is very hard, and the cost of the oracle is the least of your problems: the number of such calls grows exponentially in $n.$ Fo …
Igor Rivin's user avatar
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3 votes

Isoperimetric inequality via Crofton's formula

See the below beautiful note of Chris Croke's. Christopher B. Croke, MR 2361884 A synthetic characterization of the hemisphere, Proc. Amer. Math. Soc. 136 (2008), no. 3, 1083--1086 (electronic).]
Igor Rivin's user avatar
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11 votes
Accepted

About a solid which satisfies $\sum_{i=1}^{n}x_i=0, |x_i|\le1\ (i=1,2,\cdots,n)$

This question (in a much more general form) is answered in this preprint by Marichal and Mosinghoff. They point out that the answer to your question actually goes back to Polya's PhD thesis.
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2 votes
Accepted

Is there a general formula for calculating the volume of elliptical simplex on the surface o...

See these notes by J. G. Heckman (he focuses on the hyperbolic case, but the spherical case is essentially identical).
Igor Rivin's user avatar
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0 votes
Accepted

Surface integral approximation

This is really a question about Minkowski content. If you look at the cited article (under properties), this seems to indicate that under your Lipschitz condition, the answer to your question is YES ( …
Igor Rivin's user avatar
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7 votes
Accepted

How to partition a quadrilateral into a finite number of equal-area triangles

See this Wikipedia article.
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5 votes
Accepted

Euclidean inside Hyperbolic

I believe there is no good model of $\mathbb{E}^2$ in $\mathbb{H}^2.$ However, there is an excellent model in $\mathbb{H}^3:$ any horosphere will work. Also This is not particularly interesting, but …
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3 votes

A question concerning a well known "law" about triangles.

This is a duplicate of this question, which has good answers.
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2 votes

Escher, Conway, Kali, etc.

Can't speak for the Conway -> generators, but for drawing, there is this http://www.plunk.org/~hatch/HyperbolicApplet/ I am not sure why "rummaging" was necessary for the D. Huson program, since ther …
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2 votes

Volume ratio of $\ell_1$ balls and $\ell_1$ surfaces

I am a little confused. $S_{d-1}$ is a disjoint union of $2^d$ isometric regular simplices (one of them is the set of $x_1, \dotsc, x_d,$ with $x_i \geq 0,$ and $\sum x_i = 1.$ The volume of such a si …
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4 votes

Curvature of a finite metric space

These notes by John Lott (covering some joint work with Villani) do it for length spaces, which finite metric spaces never are, but if you join the points by edges whose lengths are the distances (so …
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4 votes

Convex hulls have longer boundaries

The magic words are: The Crofton Formula.
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3 votes
Accepted

Bound on maximum distance between points on a unit N-Sphere

Call the quantity in question $D(M, N),$ and let the volume of the spherical cap of dimension $N$ and radius $r$ $V(N, r),$normalized so that the volume of the whole sphere is $1.$ Since the caps of r …
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1 vote

Fixed points of finite order isometries of metric spaces

If $X$ is a CAT(0) space, this is true (the standard reference is Bridson-Haefliger, corr. 2.8, though the result precedes them by several decades). I believe the argument also works for $p$-uniformly …
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3 votes

conjugacy between geodesic flows on 2-tori

Yes, this is theorem C in: Croke, Christopher B.(1-PA) Rigidity for surfaces of nonpositive curvature. Comment. Math. Helv. 65 (1990), no. 1, 150–169.
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