(Abridged abstract) For a finite real reflection group W and a W-orbit O of
flats in its reflection arrangement---or equivalently a conjugacy class of its
parabolic subgroups---we introduce a statistic on elements of W. We then study
the operator of right-multiplication within the group algebra of W by the
element whose coefficients are given by this statistic.
We show that the notions of $R$-trivial monoid and weakly ordered monoid are
equivalent. We use this fact to construct a complete system of orthogonal
idempotents for all $R$-trivial monoids.
We propose a definition of an "oriented interval greedoid" that
simultaneously generalizes the notion of an oriented matroid and the
construction on antimatroids introduced by L. J. Billera, S. K. Hsiao, and J.
S. Provan in "Enumeration in convex geometries and associated polytopal
subdivisions of spheres" [Discrete Comput. Geom. 39 (2008), no. 1-3, 123--137].
As for of oriented matroids, associated to each oriented interval greedoid is a
spherical simplicial complex whose face enumeration depends only on the
underlying interval greedoid.
We propose a definition of an "oriented interval greedoid" that
simultaneously generalizes the notion of an oriented matroid and the
construction on antimatroids introduced by L. J. Billera, S. K. Hsiao, and J.
S. Provan in "Enumeration in convex geometries and associated polytopal
subdivisions of spheres" [Discrete Comput. Geom. 39 (2008), no. 1-3, 123--137].
As for of oriented matroids, associated to each oriented interval greedoid is a
spherical simplicial complex whose face enumeration depends only on the
underlying interval greedoid.