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Category Archives: Convexity
Ricky and Branko
Two dear friends, and great geometers, Ricky Pollack and Branko Grünbaum passed away a few weeks ago. Ricky was a close friend that both Mazi my wife and I loved, and we were both fascinated by his wisdom, charm and … Continue reading
Posted in Combinatorics, Convexity, Geometry, Obituary
Tagged Branko Grunbaum, Ricky Pollack
2 Comments
Nathan Rubin Improved the Bound for Planar Weak ε-Nets and Other News From Ein-Gedi
I just came back from a splendid visit to Singapore and Vietnam and I will write about it later. While I was away, Nathan Rubin organized a lovely conference on topics closed to my heart ERC Workshop: Geometric Transversals and Epsilon-Nets with … Continue reading
Basic Notions Seminar is Back! Helly Type Theorems and the Cascade Conjecture
Kazhdan’s Basic Notion Seminar is back! The “basic notion seminar” is an initiative of David Kazhdan who joined the Hebrew University math department around 2000. People give series of lectures about basic mathematics (or not so basic at times). Usually, speakers do … Continue reading
Posted in Combinatorics, Convexity, Open problems
Tagged David Kazhdan, Helly type theorems, Tverberg's theorem
4 Comments
From Oberwolfach: The Topological Tverberg Conjecture is False
The topological Tverberg conjecture (discussed in this post), a holy grail of topological combinatorics, was refuted! The three-page paper “Counterexamples to the topological Tverberg conjecture” by Florian Frick gives a brilliant proof that the conjecture is false. The proof is … Continue reading
Posted in Combinatorics, Conferences, Convexity, Updates
Tagged Florian Frick, Issac Mabillard, Uli Wagner
3 Comments
Around Borsuk’s Conjecture 3: How to Save Borsuk’s conjecture
Borsuk asked in 1933 if every bounded set K of diameter 1 in can be covered by d+1 sets of smaller diameter. A positive answer was referred to as the “Borsuk Conjecture,” and it was disproved by Jeff Kahn and me in 1993. … Continue reading
A Weak Form of Borsuk Conjecture
Problem: Let P be a polytope in with n facets. Is it always true that P can be covered by n sets of smaller diameter? I also asked this question over mathoverflow, with some background and motivation.
Around Borsuk’s Conjecture 1: Some Problems
Greetings to all! Karol Borsuk conjectured in 1933 that every bounded set in can be covered by sets of smaller diameter. In a previous post I described the counterexample found by Jeff Kahn and me. I will devote a few posts … Continue reading
The Combinatorics of Cocycles and Borsuk’s Problem.
Cocycles Definition: A -cocycle is a collection of -subsets such that every -set contains an even number of sets in the collection. Alternative definition: Start with a collection of -sets and consider all -sets that contain an odd number of members … Continue reading
Nerves of Convex Sets – A Recent Result by Martin Tancer
Martin Tancer recently found a very beautiful proof that finite projective planes can’t be represented by convex sets in any fixed dimension. This was asked in the paper entitled “Transversal numbers for hypergraphs arising in geometry” by Noga Alon, Gil … Continue reading
Posted in Convexity
2 Comments
Optimal Colorful Tverberg’s Theorem by Blagojecic, Matschke, and Ziegler
Pavle Blagojevic, Benjamin Matschke, and Guenter Ziegler settled for the case that is a prime, the “colorful Tverberg’s conjecture.” (Problem 6 in this post.) This gives a sharp version for Zivaljevic and Vrecica theorem, and crossed the “connectivity of chessboard complexes barrier”. Here is … Continue reading
Posted in Convexity
4 Comments
