BLM realization for 𝒰ℤ(𝔤𝔩 ̂n)

Abstract

In 1990, Beilinson–Lusztig–MacPherson (BLM) discovered a realization for quantum [Formula: see text] via a geometric setting of quantum Schur algebras. We will generalize their result to the classical affine case. More precisely, we first use Ringel–Hall algebras to construct an integral form [Formula: see text] of [Formula: see text], where [Formula: see text] is the universal enveloping algebra of the loop algebra [Formula: see text]. We then establish the stabilization property of multiplication for the classical affine Schur algebras. This stabilization property leads to the BLM realization of [Formula: see text] and [Formula: see text]. In particular, we conclude that [Formula: see text] is a [Formula: see text]-Hopf subalgebra of [Formula: see text]. As a bonus, this method leads to an explicit [Formula: see text]-basis for [Formula: see text], and it yields explicit multiplication formulas between generators and basis elements for [Formula: see text]. As an application, we will prove that the natural algebra homomorphism from [Formula: see text] to the affine Schur algebra over [Formula: see text] is surjective.