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A branch of geometry dealing with convex sets and functions. Polytopes, convex bodies, discrete geometry, linear programming, antimatroids, ...

41 votes
Accepted

Which polygons can be turned inside out by a smooth deformation?

This question was explored here: Lenhart, William J., and Sue H. Whitesides. "Reconfiguring closed polygonal chains in Euclidean $d$-space." Discrete & Computational Geometry 13, no. 1 (1995): 123 …
Joseph O'Rourke's user avatar
16 votes

Cutting convex sets

Permit me (as a late-comer) to add a bit more information. The 2D version of your question was posed by R. Nandakumar and N. Ramana Rao and posted at TOPP. They have written "an introduction" to thei …
Joseph O'Rourke's user avatar
13 votes

Area Enclosed by the Convex Hull of a Set of Random Points

Let $A$ be the expected area. Then: $$\lim_{n \rightarrow \infty} \frac{n}{\ln n} (1 - A) = \frac{8}{3} \;.$$ This can be found in many places, e.g., this MathWorld article. [Updated with comparison …
Joseph O'Rourke's user avatar
12 votes

Is the diameter of a centrally symmetric convex body realized by a pair of antipodal points?

I am posting this on behalf of Costin Vîlcu (with whom I've had the pleasure of coauthoring). —J.O'Rourke The posted problem in $\mathbb{R}^3$ was answered in: Y. G. Nikonorov and Y. V. Nikonorov …
Joseph O'Rourke's user avatar
11 votes

What is known about sufficient conditions for the rigidity of a convex surface?

OP: "I'm curious about more recent research in this area." Here are two relatively recent papers. Ivan visits MO, so he may answer more definitively. Izmestiev, Ivan. "Infinitesimal rigidity of co …
Joseph O'Rourke's user avatar
11 votes

Approximating a convex disk by an ellipse

Not an answer, just an illustration to accompany the question. $K$ is an isosceles triangle with base $2$ and altitude $3$ (and so area $3$). First, I mistakenly computed the ellipse $E$ of any area w …
Joseph O'Rourke's user avatar
10 votes

What is the area of the biggest open convex set inside the unit square not containing k points?

I believe this is an open problem. This 2014 paper Dumitrescu, Adrian, Sariel Har-Peled, and Csaba D. Tóth. "Minimum convex partitions and maximum empty polytopes." Scandinavian Workshop on Algori …
Joseph O'Rourke's user avatar
10 votes

Maximum area of the intersection of a parallelogram and a triangle

I don't know if this is the optimal, but an isosceles triangle with base and height $\sqrt{2}$ overlaps $2 \left(\sqrt{2}-1\right) \approx 0.828427$ when placed as below, and so improves over $\frac{ …
Joseph O'Rourke's user avatar
9 votes
Accepted

finding the most-isolated point in a high-dimensional cube

I believe you are looking for the radius of a largest empty ball among your point set, a quantity which goes under the name of dispersion. This plays a role in robotics algorithms, e.g., LaValle's boo …
Joseph O'Rourke's user avatar
7 votes

Isometric (?) embedding problem.

To follow up on Dirk's observation in the comments, here is a smoothed version of a Reuleaux triangle with $s(x)=c$, as illustrated by the dashed normal chords, which each pass through a corner of th …
Joseph O'Rourke's user avatar
7 votes

Largest possible volume of the convex hull of a curve of unit length

Here is an image of the optimal open convex curve. Taken from Open Problems from CCCG 2012, based on this paper, which cites Nudel'man (1975): Paolo Tilli. "Isoperimetric inequalities for conve …
Joseph O'Rourke's user avatar
6 votes
Accepted

How to find the minimum number of hyperplanes to define a convex hull?

If you search for detection of redundant constraints in linear programming you will find many hits, including one to an MO question, "Detection of Redundant Constraints." One source paper is J. Go …
Joseph O'Rourke's user avatar
6 votes

Convex Polygon - Splitting into Two Congruent Pieces

There exist convex quadrilaterals which have no such splitting. And there is an $O(n^3)$ algorithm to decide if such a splitting exists for a (nonconvex) $n$-gon. See the paper by Dania El-Khechen, Th …
Joseph O'Rourke's user avatar
6 votes

Do random projections (approximately) preserve convexity?

Not an answer, just a thought. It seems a bit too much to hope for. Here are 300 points randomly sprinkled on the surface of an ellipsoid in $\mathbb{R}^3$, and then projected to two dimensions. No m …
Joseph O'Rourke's user avatar
5 votes

Helly's theorem in other areas of mathematics

Helly's theorem plays a role in economics theory and in game theory (noncooperative games): Fuchs-Seliger, Susanne. "An application of Helly's theorem to preference-generated choice corresponde …
Joseph O'Rourke's user avatar

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