Infinite-dimensional reductive monoids associated to highest weight representations of Kac–Moody groups

Abstract Starting with a highest weight representation of a Kac–Moody group over the complex numbers, we construct a monoid whose unit group is the image of the Kac–Moody group under the representation, multiplied by the nonzero complex numbers. We show that this monoid has similar properties to those of a J -irreducible reductive linear algebraic monoid. In particular, the monoid is unit regular and has a Bruhat decomposition, and the idempotent lattice of the generalized Renner monoid of the Bruhat decomposition is isomorphic to the face lattice of the convex hull of the Weyl group orbit of the highest weight.