Tag Archives: Oberwolfach

Richard Ehrenborg’s problem on spanning trees in bipartite graphs

Posted on September 1, 2019 by Gil Kalai

Richard Ehrenborg with a polyhedron In the Problem session last Thursday in Oberwolfach, Steve Klee presented a beautiful problem of Richard Ehrenborg regarding the number of spanning trees in bipartite graphs. Let be a bipartite graph with vertices on one … Continue reading →

Posted in Combinatorics, Open problems | Tagged , | 4 Comments

Mohammad Ghomi and Joel Spruck settled the Cartan-Hadamard conjecture!

Posted on August 28, 2019 by Gil Kalai

Greetings from Oberwolfach from a great conference on algebraic, geometric, and topological combinatorics. Stay tuned for more pictures and updates from Oberwolfach and CERN, and also in case you did not see it already here is the link to the … Continue reading →

Posted in Geometry | Tagged , , , | 5 Comments

From Oberwolfach: The Topological Tverberg Conjecture is False

Posted on February 6, 2015 by Gil Kalai

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 , , , | 3 Comments

My Mathematical Dialogue with Jürgen Eckhoff

Posted on July 3, 2014 by Gil Kalai

Jürgen Eckhoff, Ascona 1999 Jürgen Eckhoff is a German mathematician working in the areas of convexity and combinatorics. Our mathematical paths have met a remarkable number of times. We also met quite a few times in person since our first … Continue reading →

Posted in Combinatorics, Convex polytopes, Open problems | Tagged , , , , , , | 1 Comment

Amazing: Peter Keevash Constructed General Steiner Systems and Designs

Posted on January 16, 2014 by Gil Kalai

Here is one of the central and oldest problems in combinatorics: Problem: Can you find a collection S of q-subsets from an n-element set X set so that every r-subset of X is included in precisely λ sets in the collection? … Continue reading →

Posted in Combinatorics, Open problems | Tagged , , , | 16 Comments