The next informal seminar will be given by our own Patrick Weber. His title is *“From complex to hypercomplex cohomologies”*. The talk will take place at 2pm in the Salle de Profs (9th floor of building NO). The abstract is below. [Read more…]

## Roman Golovko – an introduction to Legendrians 20th September

The next informal seminar will be given by Roman Golovko, with the title *“A gentle introduction to Legendrian submanifolds towards Arnold-type estimates”*. The talk will take place on Tuesday 20th September 10-12, in the Salle de Profs, (mathematics department, on the 9th floor of building NO, Campus de la Plaine). Roman’s abstract is below. [Read more…]

## Introduction to twistor spaces, on September 13

The reading seminar will kick off this term with a series of informal talks in which people will introduce the background of the various areas they are working on. The first talk will be give by me (Joel) at 11am on Tuesday 13th September, in the Salle de Profs, 9th floor of building NO. The abstract is below. If you would like to give a talk in this seminar then please email me, I’m always looking for volunteers! (Also note that in a while, the reading seminar will switch to focus on a single topic for several talks.)

Given an oriented Riemannian 4-manifold M, its twistor space is a certain 2-sphere bundle Z, over M. Z caries two natural almost complex structures and features of the Riemannian geometry of M can be translated into the almost complex geometry of Z. Traditionally, the focus is on the case when one of these almost complex structures makes Z a genuine complex manifold. This happens when the starting metric is anti-self-dual (part of the curvature tensor vanishes). Then one can use techniques of complex geometry to answer questions about M. More recently symplectic techniques have found applications. There is a natural closed 2-form on Z which sometimes tames one of the two almost complex structures. This corresponds to a curvature

inequality for M. I will try and describe both the complex and symplectic approaches to twistor spaces, giving a few of the details along the way.

## Lower Ricci working group, talk 6

On 30th March, 10:30-11:30, in the Salle de Profs, Bruno will give the 6th talk in our working group on manifolds with lower bounds on Ricci curvature. He will describe some of the following topics, from Chapter 9 of Cheeger’s notes: almost volume cones and almost metric cones, Reifenberg’s method, tangent cones at infinity.