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

29 votes
Accepted

How does one find the "loneliest person on the planet"?

The paper Vaidya, Pravin M., An $O(n \log n)$ algorithm for the all-nearest-neighbors problem, Discrete Comput. Geom. 4, No. 2, 101-115 (1989), ZBL0663.68058 gives an $O(n \log n)$ algorithm for the " …
Reid Barton's user avatar
  • 25.2k
24 votes

Algorithm for finding the volume of a convex polytope

I think this problem is hard--the known algorithms are both slow and nontrivial to implement. See Exact Volume Computation for Polytopes for a survey. An interesting feature is that there are variou …
Reid Barton's user avatar
  • 25.2k
18 votes

Is it possible to capture a sphere in a knot?

The regular dodecahedron is not a local minimum for total edge length either. Consider the five vertices Vi, i = 1, ..., 5 of a face together with the "center" C of the face. The spherical triangle …
Reid Barton's user avatar
  • 25.2k
13 votes

Upper bound on the area of a midpoint pentagon?

I used qepcad to compute that the intersection of the set of possible area ratios with the interval [1/2, 3/4] is (1/2, 3/4). Since the set of possible area ratios is the image of a connected space u …
Reid Barton's user avatar
  • 25.2k
8 votes

How to compute the average distance till intersection within a triangle in $\mathbb{R}^2$?

If you call the random variable you described X, then it's much easier to compute the expected value of X2. Indeed, for any point P inside triangle ABC, the expected value of X2 where X is the length …
Reid Barton's user avatar
  • 25.2k
5 votes

Feasibility of a list of prescribed distances in R^3

Embedded in Anton's answer are the following two aspects of the problem: You first need to decide (or be given) the combinatorial structure of the graph you want to embed, i.e., assign the n distanc …
Reid Barton's user avatar
  • 25.2k
1 vote
Accepted

Deflating a tetrahedron to a $K_4$ graph with equal changes to sidelengths

Yes, this is true. The main point is that the "first thing that goes wrong" cannot be two vertices coming together. Let $a_0$, $b_0$, $c_0$, $d_0$, $e_0$, $f_0$ denote the edge lengths of the original …
Reid Barton's user avatar
  • 25.2k