organized by Igor Krichever and Andrei Okounkov

Mondays, 5:30, Room 507 (except Oct. 21 and 28, when it will meet in
room 622, 6pm-7:30pm)

Schedule of talks for Fall 2013:

Sept 23 |
Staff |
Lie algebras and Kac polynomials |

Oct 7, 6:00 |
Davesh Maulik |
Motivic residues (note special time!) |

Oct 14 |
Maria Angelica Cueto |
Faithful tropicalization of the Grassmannian
of planes |

Oct 21, 6:00 in 622 |
Hiraku Nakajima |
Kazhdan-Lusztig conjecture via quasi-maps and
Uhlenbeck spaces |

Oct 28, 6:00 in 622 |
Sachin Gautam |
Analytic construction of the R-matrix of
Yangians |

Nov 11 |
Clément Hongler |
Planar Ising model with magnetic field and E8
structure |

Nov 18 |
Daniel Halpern-Leistner |
Variation of geometric invariant theory
quotients and autoequivalences of derived categories |

Nov 25 |
Sachin Gautam |
From dynamical difference equations to Casimir connections
(after M. Balagović) |

Dec 2 |
Daniel Shenfeld |
Abelianization of Stable Envelopes in
Symplectic Resolutions |

Abstracts

October 14Fix a complete non-Archimedean valued field K. Any subscheme X of (K^*)^n can be "tropicalized" by taking the (closure) of the coordinate-wise valuation. This process is highly sensitive to coordinate changes. When restricted to group homomorphisms between the ambient tori, the image changes by the corresponding linear map. This was the foundational setup of tropical geometry.

In recent years the picture has been completed to a commutative diagram including the analytification of X in the sense of Berkovich. The corresponding tropicalization map is continuous and surjective and is also coordinate-dependent. Work of Payne shows that the Berkovich space X^an is homeomorphic to the projective limit of all tropicalizations. A natural question arises: given a concrete X, can we find a split torus containing it and a continuous section to the tropicalization map? If the answer is yes, we say that the tropicalization is faithful. The curve case was worked out by Baker, Payne and Rabinoff.

In this talk, we show that the tropical projective Grassmannian of planes is homeomorphic to a closed subset of the analytic Grassmannian in Berkovich sense. Our proof is constructive and it relies on the combinatorial description of the tropical Grassmannian as a space of (generalized) phylogenetic trees by Speyer-Sturmfels. We also show that both sets have piecewiselinear structures that are compatible with our homeomorphism.

This is joint work with M. Haebich and A. Werner (arXiv:1309.0450).

We construct integral forms of enveloping algebras, W-algebras

etc on equivariant intersection cohomology groups of quasi-maps,
Uhlenbeck

spaces, etc. Then their character formulae are deduced from
intersection

cohomology groups of fixed point sets.

The R-matrix R(u) was constructed by Drinfeld as a formal series in
1/u with coefficients from the tensor square of a Yangian. It
satisfies the Yang--Baxter equation.

In this talk I will discuss the analytic properties of this formal
series once evaluated on a pair of finite-dimensional representations.
The results presented in this talk were obtained jointly with V.
Toledano Laredo

The Ising model is a simple and natural model for ferromagnetism. In
two dimensions, it has many symmetries, on lattice level, and in
the continuum limit (by Conformal Field Theory, Schramm-Loewner
Evolution). It was suggested by Al. Zamolodchikov that the 2D Ising
model with a magnetic field can be formulated in terms of an
scattering theory with 8 types of particles. Numerical simulations are
in perfect agreement with Zamolodchikov's prediction and recently
there was experimental confirmation of this claim.

I will give an overview about what is known about this story, which
involves complex analysis and probability, Conformal Field Theory
(minimal models, W-algebras, coset models, etc), S-Matrix theory. Then
I will outline what seems to be a reasonable strategy to make rigorous
sense of it.

Based (mostly) on joint work in progress with Kalle Kytölä.

Geometric invariant theory provides a method of taking a variety with a reductive group action and producing a new "quotient" variety. I will describe a situation in which the derived category of coherent sheaves of the resulting GIT quotient has autoequivalences which don't come from automorphisms of the variety itself. These autoequivalences are always of a special form generalizing the notion of a spherical twist.

Stable envelopes, introduced by Maulik and Okounkov, form a basis for
the equivariant cohomology of symplectic resolutions. We study the
case of Nakajima quiver varieties, and relate the stable basis to that
of the associated quotient by a maximal torus, obtaining a formula for
the transition between the stable basis and the fixed point basis. As
an application, we compute the transition matrix explicitly for the
Hilbert scheme of points on the plane and obtain a formula expressing
Schur polynomials in terms of Jack polynomials.