A Monoid for the Universal K-Bruhat Order

Structure constants for the multiplication of Schubert polynomials by Schur symmetric polynomials are known to be related to the enumeration of chains in a new partial order on S_\infty, which we call the universal k-Bruhat order. Here we present a monoid M for this order and show that $M$ is analogous to the nil-Coxeter monoid for the weak order on S_\infty. For this, we develop a theory of reduced sequences for M. We use these sequences to give a combinatorial description of the structure constants above. We also give combinatorial proofs of some of the symmetry relations satisfied by these structure constants.