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

41 votes
6 answers
2k views

Approximating a convex disk by an ellipse

For every convex compact set $K$ of area $1$ in $\mathbb{R}^2$, among all ellipses of area $1$ there exists an ellipse $E$ such that the area of the symmetric difference between $K$ and $E$ is smalles …
Wlodek Kuperberg's user avatar
8 votes
2 answers
282 views

An affine characterization of ellipsoids?

Let $K$ be a convex body of volume 1 in $\mathbb{R}^n$ and $x$ a (variable) point on the boundary of $K$. Define $f_K(x)$ to be the volume of the convex hull of the union of $K$ with its reflection in …
Wlodek Kuperberg's user avatar
5 votes
1 answer
161 views

Cutting a convex body into two congruent pieces

This question is related to How to make a sandwich from just one piece of bread?, asked on Feb 23 '17 by erz, and it goes as follows: Question. If a convex closed and bounded region $C$ in the pl …
Wlodek Kuperberg's user avatar
6 votes
2 answers
205 views

Splitting the $n$-cube into two small congruent convex halves

The diameter of a bounded set is the supremum of the distances between any two points of the set, and the circumradius is the infimum of the radii of balls containing the set. Obviously, the diameter …
Wlodek Kuperberg's user avatar
28 votes
0 answers
545 views

Can every 3-dimensional convex body be trapped in a tetrahedral cage?

Can every 3-dimensional convex body be trapped in a tetrahedral cage? Although the question is fairly unambiguous, I give all relevant definitions: $\bullet$ A subset $C$ of $\mathbb{R}^n$ is an $n$-d …
Wlodek Kuperberg's user avatar
16 votes
1 answer
583 views

(A question about)${}^3$ 3-dimensional convex bodies

Related to the questions mathoverflow.net question No. 137850 and mathoverflow.net question No. 39127, is there a 3-dimensional convex body other than a ball whose perpendicular projections in all dir …
Wlodek Kuperberg's user avatar
16 votes
2 answers
1k views

Maximum area of the intersection of a parallelogram and a triangle

How large can the intersection of a parallelogram (or a square, if you prefer) with a triangle be, if each of them is of unit area? It is easy to see that the intersection can be of area 3/4 – is this …
Wlodek Kuperberg's user avatar
7 votes
1 answer
317 views

Minimum area of the convex hull of the union of a parallelogram and a triangle

This question is somewhat dual to my previously stated question about Maximum area of the intersection of a parallelogram and a triangle, where the triangle and parallelogram each is assumed to be of …
Wlodek Kuperberg's user avatar