Analysis Seminar

Speaker: Rasul Shafikov (Western)

"Lagrangian immersions, polynomial convexity, and Whitney umbrellas, III"

Time: 14:40

Room: MC 107

This is a continuation of the talk from October 11. Details will be given of the proof that a Lagrangian surface $X\subset \mathbb C^2$ near an isolated singularity which is a Whitney umbrella is locally polynomially convex. In this talk I will discuss Bruno's construction of normal forms for the dynamical system that determines the phase portrait of the characteristic foliation.

Colloquium

Speaker: David McKinnon (Waterloo)

"Rational points repel each other, so they can't be densely packed"

Time: 15:30

Room: MC 107

Well, actually, we only think that rational points repel each other, so the title is merely conjectural in general. In particular, there is a conjecture of Batyrev and Manin that says that on certain kinds of algebraic varieties, points with rational coordinates are not very densely packed. There is another conjecture, due to Vojta, that says that subvarieties of algebraic varieties cannot be approximated too well by rational points. The purpose of my talk will be to make all these notions precise, to explain what they have to do with one another, and to explain why Vojta's conjecture sometimes implies that of Batyrev and Manin.

Noncommutative Geometry

Speaker: Masoud Khalkhali (Western)

"Theorema Egregium and Gauss-Bonnet Theorem for Surfaces (2)"

Time: 10:30

Room: MC 108

Since this year we shall be busy with curavture in noncommutative geometry, I thought I should start with the most fundamental classical incarnation of this notion: Gauss' theory of curvature for surfaces, and what it can teach us. All are welcome!

When Gauss, in his celebrated paper of 1827, {\it Disquisitiones generales circa superficies curvas} {(\it General investigations of curved surfaces)} after a long series of calculations

eventually showed that the extrinsically defined curvature of a

surface can be expressed entirely in terms of its intrinsic metric (= the

first fundamental form), he got so excited that he called the obvious corollary of this result Theorema Egregium (The Remarkable Theorem).

Gauss's formidable curvature formula, and the closely related {\it Gauss-Bonnet theorem} is the foundation stone for

all of differential geometry, as it was later shown by Riemann in 1859

that the curvature of higher dimensional manifolds can be understood

purely in terms of curvatures of its two dimensional submanifolds.

Theorema Egregium can also be regarded as the infinitesimal form of,

and in fact is equivalent to, the celebrated Gauss-Bonnet Theorem. This paper of Gauss is the single most important work in the entire history of differential geometry.

Algebra Seminar

Speaker: Masoud Khalkhali & Farzad Fathizadeh (Western and York)

"Curvature in noncommutative geometry II"

Time: 15:40

Room: MC 107

In this talk, I will continue the lecture given by Masoud Khalkhali on our recent joint work on the Gauss-Bonnet theorem and scalar curvature for the noncommutative two torus, in the context of Alain Connes' noncommutative differential geometry. I will first construct the Connes-Tretkoff spectral triple encoding the metric information on this $C^*$-algebra so that we view it as a noncommutative Riemannian manifold equipped with a general metric. Then I will recall a spectral definition for its scalar curvature, and will illustrate the process of finding a local expression for the curvature by employing a special case of Connes' pseudodifferential calculus for $C^*$-dynamical systems by means of which one can pursue the heat kernel scheme of elliptic differential operators and index theory. I should mention that recently Connes and Moscovici also found precisely the same formula independently. At the end I will explain how this formula fits into our earlier work which extends the Gauss-Bonnet theorem of Connes and Tretkoff to general conformal structures on noncommutative two tori.