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Tag Archives: Topological combinatorics
Polymath10, Post 2: Homological Approach
We launched polymath10 a week ago and it is time for the second post. In this post I will remind the readers what the Erdos-Rado Conjecture and the Erdos-Rado theorem are, briefly mention some points made in the previous post and in … Continue reading
Polymath3 (PHC6): The Polynomial Hirsch Conjecture – A Topological Approach
This is a new polymath3 research thread. Our aim is to tackle the polynomial Hirsch conjecture which asserts that there is a polynomial upper bound for the diameter of graphs of -dimensional polytopes with facets. Our research so far was … Continue reading
Posted in Convex polytopes, Geometry, Polymath3
Tagged Hirsch conjecture, Polymath3, Topological combinatorics
37 Comments
A Beautiful Garden of Hypertrees
We had a series of posts (1,2,3,4) “from Helly to Cayley” on weighted enumeration of Q-acyclic simplicial complexes. The simplest case beyond Cayley’s theorem were Q-acyclic complexes with vertices, edges, and triangles. One example is the six-vertex triangulation of the … Continue reading
Posted in Combinatorics
Tagged Mishael Rosenthal, Nati Linial, Roy Meshulam, Topological combinatorics, Trees
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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
Tagged Cayley theorem, Helly type theorems, Topological combinatorics
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
Tagged Cayley theorem, Helly Theorem, Simplicial complexes, Topological combinatorics, Trees
9 Comments
A Small Debt Regarding Turan’s Problem
Turan’s problem asks for the minimum number of triangles on n vertices so that every 4 vertices span a triangle. (Or equivalently, for the maximum number of triangles on n vertices without a “tetrahedron”, namely without having four triangles on … Continue reading
