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arXiv · 1303.3955

On Algebraic Semigroups and Monoids, II

Michel Brion
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Abstract

Consider an algebraic semigroup $S$ and its closed subscheme of idempotents, $E(S)$. When $S$ is commutative, we show that $E(S)$ is finite and reduced; if in addition $S$ is irreducible, then $E(S)$ is contained in a smallest closed irreducible subsemigroup of $S$, and this subsemigroup is an affine toric variety. It follows that $E(S)$ (viewed as a partially ordered set) is the set of faces of a rational polyhedral convex cone. On the other hand, when $S$ is an irreducible algebraic monoid, we show that $E(S)$ is smooth, and its connected components are conjugacy classes of the unit group.

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Michel Brion. 2013-12-20. On Algebraic Semigroups and Monoids, II. https://arxiv.org/abs/1303.3955

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