A Quasisymmetric Analogue of the Bruhat Order

Abstract

We define a quasisymmetric analogue of the Bruhat order on indexed binary forests using the multiplicity-free Monk rule for forest polynomials. For the finite window $\mathcal{P}_n$ of forests supported in $[1,n]$, we construct an explicit partition into saturated chains terminating at the top rank, proving that $\mathcal{P}_n$ is strongly Sperner. We also recover the known Catalan ballot formula for its rank numbers. For principal lower intervals, we prove that $[\varnothing,F]$ is a lattice if and only if it is Boolean, and characterize this property by the avoidance of a bent corner in $F$, a path of three internal vertices that changes direction. This gives a forest analogue of the pattern-avoidance characterization of Boolean principal Bruhat intervals. We also conjecture that every principal lower interval is shellable, providing computational evidence and proving the conjecture for a family of hook intervals.