Equivariant $K$-theory ring of the affine Grassmannian as deformation to normal cone

Abstract

We revisit fixed-point localization techniques in equivariant topological $K$-theory, presenting its fibers over varying points of the equivariant base as cohomology of the corresponding fixed-point schemes. We then employ this framework for an effective description of the equivariant $K$-theory ring of the affine Grassmannian $\mathcal{G}r_G$. We do so by a detailed study of the relevant fixed points, extending known results to the action of any $(x, ζ) \in T \times \mathbb{G}_{m}^{\mathrm{rot}}$. We obtain a description of the complexified topological $K$-theory ring $K^{\mathrm{top}, 0}_{T\times \mathbb{G}_{m}^{\mathrm{rot}}}(\mathcal{G}r_G; \mathbb{C})$ as the ring of functions on a family of affine blowups, extending and proving a conjecture of Roman Bezrukavnikov. While this ring is fairly big, we provide a preferred infinite set of topological generators.