Zero loci of Skew-growth functions for dual Artin monoids

We show that the skew-growth function of a dual Artin monoid of finite type P has exactly rank(P)=:l simple real zeros on the interval (0,1]. The proofs for types Al and Bl are based on an unexpected fact that the skew-growth functions, up to a trivial factor, are expressed by Jacobi polynomials due to a Rodrigues type formula in the theory of orthogonal polynomials. The skew-growth functions for type Dl also satisfy Rodrigues type formulae, but the relation with Jacobi polynomials is not straightforward, and the proof is intricate. We show that the smallest root converges to zero as the rank l tends to infinity.