On the Borel submonoid of a symplectic monoid

Abstract

In this article, we study the Bruhat-Chevalley-Renner order on the complex symplectic monoid MSpn. After showing that this order is completely determined by the Bruhat-Chevalley-Renner order on the linear algebraic monoid of n×n matrices Mn, we focus on the Borel submonoid of MSpn. By using this submonoid, we introduce a new set of type B set partitions. We determine their count by using the “folding” and “unfolding” operators that we introduce. We show that the Borel submonoid of a rationally smooth reductive monoid with zero is rationally smooth. Finally, we analyze the nilpotent subsemigroups of the Borel semigroups of Mn and MSpn. We show that, contrary to the case of MSpn, the nilpotent subsemigroup of the Borel submonoid of Mn is irreducible.