Abstract:
This is a historical talk about the recent confluence of two lines of research in equivariant elliptic cohomology, one concerned with connected Lie groups, the other with the finite case. These themes come together in (what seems to me remarkable) work of N. Ganter, relating replicability of McKay-Thompson series to the theory of exponential cohomology operations.

Abstract:
We address the problem of defining Schubert classes independently of a reduced word in equivariant elliptic cohomology, based on the Kazhdan-Lusztig basis of a corresponding Hecke algebra. We study some basic properties of these classes, and make two important conjectures about them: a positivity conjecture, and the agreement with the topologically defined Schubert classes in the smooth case. We prove some special cases of these conjectures.

Abstract:
Equivariant elliptic cohomology with complex coefficients was defined axiomatically by Ginzburg, Kapranov and Vasserot and constructed by Grojnowski. We give an invariant definition of S^1-equivariant elliptic cohomology, and use it to give an entirely cohomological proof of the rigidity theorem of Witten for the elliptic genus. We also state and prove a rigidity theorem for families of elliptic genera.

Abstract:
In this paper we explain the parallelism in the classification of three different kinds of mathematical objects: (i) Classical r-matrices. (ii) Generalized cohomology theories that have Chern classes for complex vector bundles. (iii) 1-dimensional formal groups. The main point of the paper is a construction of the elliptic algebra associated to Belavin's classical elliptic r-matrix in terms of Equivariant elliptic cohomology of the Steinberg varieties associated to some partial flag manifolds.

Abstract:
We construct a Thom class in complex equivariant elliptic cohomology extending the equivariant Witten genus. This gives a new proof of the rigidity of the Witten genus, which exhibits a close relationship to recent work on non-equivariant orientations of elliptic spectra.

Abstract:
We apply equivariant elliptic cohomology to the Steinberg variety in Springer theory, and prove that the corresponding convolution algebra is isomorphic to the elliptic affine Hecke algebra constructed by Ginzburg-Kapranov-Vasserot. Under this isomorphism, we describe explicitly the cohomology classes that correspond to the elliptic Demazure-Lusztig operators. As an application, we study the Deligne-Langlands theory in the elliptic setting, and classify irreducible representations of the elliptic affine Hecke algebra. The irreducible representations are in one to one correspondence with certain nilpotent Higgs bundles on the elliptic curve. We also study representations at torsion points in type-$A$.

Abstract:
For each elliptic curve A over the rational numbers we construct a 2-periodic S^1-equivariant cohomology theory E whose cohomology ring is the sheaf cohomology of A; the homology of the sphere of the representation z^n is the cohomology of the divisor A(n) of points with order dividing n. The construction proceeds by using the algebraic models of the author's AMS Memoir ``Rational S^1 equivariant homotopy theory.'' and is natural and explicit in terms of sheaves of functions on A. This is Version 5.2 of a paper of long genesis (this should be the final version). The following additional topics were first added in the Fourth Edition: (a) periodicity and differentials treated (b) dependence on coordinate (c) relationship with Grojnowksi's construction and, most importantly, (d) equivalence between a derived category of O_A-modules and a derived category of EA-modules. The Fifth Edition included (e) the Hasse square and (f) explanation of how to calculate maps of EA-module spectra.

Abstract:
Given a simple simply connected compact Lie group G and a G-space M, we study the quantization of the category of parametrized positive energy representations of the loop group LG at a fixed positive level. This procedure is described in terms of dominant K-theory of the loop group evaluated on the space of basic classical gauge fields about a circle, for families of two dimensional conformal field theories parametrized over M. More concretely, we construct a holomorphic sheaf over a universal elliptic curve with values in dominant K-theory of the loop space LM, and show that each stalk of this sheaf is a cohomological functor of M. We also interpret this theory as a model of equivariant elliptic cohomology of M as constructed by Grojnowski.

Abstract:
We construct a canonical Thom isomorphism in Grojnowski's equivariant elliptic cohomology, for virtual T-oriented T-equivariant spin bundles with vanishing Borel-equivariant second Chern class, which is natural under pull-back of vector bundles and exponential under Whitney sum. It extends in the complex-analytic case the non-equivariant sigma orientation of Hopkins, Strickland, and the author. The construction relates the sigma orientation to the representation theory of loop groups and Looijenga's weighted projective space, and sheds light even on the non-equivariant case. Rigidity theorems of Witten-Bott-Taubes including generalizations by Kefeng Liu follow.

Abstract:
We use the geometry of 2|1-dimensional gauged sigma models to construct cocycles for the twisted equivariant elliptic cohomology with complex coefficients of a smooth manifold with an action by a finite group. Our twists come from classical Chern-Simons theory of the group, and we construct induction functors using Freed--Quinn quantization of finite gauge theories.