Affine Gindikin-Karpelevich formula via Uhlenbeck spaces

TL;DR

This paper proves a geometric version of the affine Gindikin-Karpelevich formula for untwisted affine Kac-Moody groups over local fields of positive characteristic, explaining differences from the finite-dimensional case through intersection cohomology of Uhlenbeck spaces.

Contribution

It provides a geometric proof of the affine Gindikin-Karpelevich formula using intersection cohomology of Uhlenbeck-type moduli spaces, clarifying the additional terms compared to the finite case.

Findings

Established the affine Gindikin-Karpelevich formula in positive characteristic.

Connected the additional terms to the geometry of loop groups and algebraic surfaces.

Provided a geometric explanation for differences between finite and affine cases.

Abstract

We prove a version of the Gindikin-Karpelevich formula for untwisted affine Kac-Moody groups over a local field of positive characteristic. The proof is geometric and it is based on the results of [1] about intersection cohomology of certain Uhlenbeck-type moduli spaces (in fact, our proof is conditioned upon the assumption that the results of [1] are valid in positive characteristic). In particular, we give a geometric explanation of certain combinatorial differences between finite-dimensional and affine case (observed earlier by Macdonald and Cherednik), which here manifest themselves by the fact that the affine Gindikin-Karpelevich formula has an additional term compared to the finite-dimensional case. Very roughy speaking, that additional term is related to the fact that the loop group of an affine Kac-Moody group (which roughly speaking should be thought of as some kind of "double loop group") does not behave well from algebro-geometric point of view; however it has a better behaved version which has something to do with algebraic surfaces. A uniform (i.e. valid for all local fields) and unconditional (but not geometric) proof of the affine Gindikin-Karpelevich formula is going to appear in [2].