# Category Archives: Convexity

## Sarkaria’s Proof of Tverberg’s Theorem 1

Helge Tverberg Ladies and gentlemen, this is an excellent time to tell you about the beautiful theorem of Tverberg and the startling proof of Sarkaria to Tverberg’s theorem (two parts). A good place to start is Radon’s theorem. 1. The theorems of Radon, … Continue reading

Posted in Convexity | | 16 Comments

## Helly, Cayley, Hypertrees, and Weighted Enumeration III

This is the third and last part of the journey from a Helly type conjecture of Katchalski and Perles to a Cayley’s type formula for “hypertrees”.  (On second thought I decided to divide it into two devoting the second to probabilistic questions.) … Continue reading

Posted in Combinatorics, Convexity, Open problems, Probability | 7 Comments

## Helly’s Theorem, “Hypertrees”, and Strange Enumeration II: The Formula

In the first part of this post we discussed an appealing conjecture regaring an extension of Cayley’s counting trees formula. The number of d-dimensional “hypertrees” should somehow add up  to . But it was not clear to us which complexes we want … Continue reading

Posted in Combinatorics, Convexity | | 6 Comments

## Helly’s Theorem, “Hypertrees”, and Strange Enumeration I

1. Helly’s theorem and Cayley’s formula Helly’s theorem asserts: For a family of n convex sets in , n > d, if every d+1 sets in the family have a point in common then all members in the family have a point in common. … Continue reading

Posted in Combinatorics, Convexity | | 9 Comments

## Five Open Problems Regarding Convex Polytopes

The problems  1. The conjecture A centrally symmetric d-polytope has at least non empty faces. 2. The cube-simplex conjecture For every k there is f(k) so that every d-polytope with has a k-dimensional face which is either a simplex … Continue reading