Path representation of maximal parabolic Kazhdan-Lusztig polynomials

We provide simple rules for the computation of Kazhdan--Lusztig polynomials in the maximal parabolic case. They are obtained by filling regions delimited by paths with "Dyck strips" obeying certain rules. We compare our results with those of Lascoux and Sch\"utzenberger.Comment: v3: fixed proof of lemma