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 - the authors introduce a statistic noninv O (w) on
w in W that counts the number of O -noninversions of w . This
generalises the classical (non-)inversion statistic for permutations w
in the symmetric group S n. The authors then study the operator ? O of
right-multiplication within the group algebra CW by the element that has
noninv O (w) as its coefficient on w.