Extended Weyl groups, Hurwitz transitivity and weighted projective lines II: The wild case

Abstract: We study extended Weyl groups of domestic and wild type. We start with an extended Coxeter-Dynkin diagram, attach to it a set of roots (vectors) in an $\mathbb{R}$-space and define the extended Weyl groups as groups that are generated by the reflections related to these roots, the so called simple reflections. We relate to such a group $W$ a generalized root system, and determine the structure of $W$. In particular we present a normal form for the elements of $W$. Further we define Coxeter transformations in $W$, and show the transitivity of the Hurwitz action on the set of reduced reflection factorizations of a Coxeter transformation where the reflections are the conjugates of the simple reflections in $W$. We give two applications of our results. In the context of representation theory of algebras, we establish an isomorphism between the poset of thick subcategories that are generated by exceptional sequences of a hereditary connected ext-finite abelian $k$-category with a tilting object and the poset of elements in the extended Weyl group that are below a Coxeter transformation with respect to the absolute order. The second application concerns the theory of unimodal singularities. In particular, we provide an answer to a question of Brieskorn for the classical monodromy operator in the case of hyperbolic singularities.