Monthly Archives: June 2009

The topological Cauchy-Schwarz inequality

I recently made the final edits to my paper “Positivity of the universal pairing in 3 dimensions”, written jointly with Mike Freedman and Kevin Walker, to appear in Jour. AMS. This paper is inspired by questions that arise in the … Continue reading

Posted in 3-manifolds, TQFT | Tagged , , , , , , , , , , , , , , , , | 3 Comments

Big mapping class groups and dynamics

Mapping class groups (also called modular groups) are of central importance in many fields of geometry. If is an oriented surface (i.e. a -manifold), the group of orientation-preserving self-homeomorphisms of is a topological group with the compact-open topology. The mapping … Continue reading

Posted in Dynamics, Groups | Tagged , , , , , , , , , , , | 1 Comment

Measure theory, topology, and the role of examples

Bill Thurston once observed that topology and measure theory are very immiscible (i.e. they don’t mix easily); this statement has always resonated with me, and I thought I would try to explain some of the (personal, psychological, and mathematical) reasons … Continue reading

Posted in Psychology | Tagged , , , , , , , , , , | 3 Comments

Round slices of pointy objects

A regular tetrahedron (in ) can be thought of as the convex hull of four pairwise non-adjacent vertices of a regular cube. A bisecting plane parallel to a face of the cube intersects the tetrahedron in a square (one can … Continue reading

Posted in Convex geometry | Tagged , , , | Leave a comment

Ellipsoids and KAK

As many readers are no doubt aware, the title of this blog comes from the famous book Geometry and the Imagination by Hilbert and Cohn-Vossen (based on lectures given by Hilbert). One of the first things discussed in that book … Continue reading

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Combable functions

The purpose of this post is to discuss my recent paper with Koji Fujiwara, which will shortly appear in Ergodic Theory and Dynamical Systems, both for its own sake, and in order to motivate some open questions that I find … Continue reading

Posted in Ergodic Theory | Tagged , , , , , , | 2 Comments

Quasimorphisms and laws

A basic reference for the background to this post is my monograph. Let be a group, and let denote the commutator subgroup. Every element of can be expressed as a product of commutators; the commutator length of an element is the … Continue reading

Posted in Groups | Tagged , , , , | 3 Comments