L2

In the year 1858, Herman Ludwig Ferdinand von Helmholtz published in Crelle's Journal a deep and pioneering paper on vortex motions where the topological properties of vortex lines in a fluid motion were emphasised. This work has been a strong source of inspiration for P G Tait who settled down the foundation of knot theory and for H Poincare, the father of geometric theory of dynamical systems. As a matter of fact, by the end of the 19^th century, three topics, knots, flows and fluids were closely related. In the last decades, the topic has been boosted by a series of new appealing problems and interesting results gathered under the name Topological Methods in Hydrodynamics. Our talk will start with a short trip around the pioneering works. Then we will focus on two essential recent topics:

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The recent developments of Mathematical Physics have brought very new ideas about the nature of space . I will argue that we have to mix the methods of noncommutative geometry of Alain Connes with the prophetic views of Grothendieck about the so-called motives and their motivic Galois group .
The dream of a "cosmic Galois group" may soon become an established reality .
 

I will give an expository account of Mathai, Melrose, and Singer [math.DG/0402329], explaining how to define the projective Dirac "operator" when the underlying manifold is neither spin nor spin_C, and how to define its analytic index which need not be an integer. Nevertheless, the usual index formulas apply. Professor Singer will be admitted as honorary member of the London Mathematical Society just before his talk.