Springer correspondences for dihedral groups

Recent work by a number of people has shown that complex reection groups give rise to many representation-theoretic structures (e.g., generic degrees and families of characters), as though they were Weyl groups of algebraic groups. Conjecturally, these structures are actually describing the representation theory of as-yet undescribed objects called spetses, of which reductive algebraic groups ought to be a special case. In this paper we carry out the Lusztig-Shoji algorithm for calculating Green functions for the dihedral groups. With a suitable set-up, the output of this algorithm turns out to satisfy all the integrality and positivity conditions that hold in the Weyl group case, so we may think of it as describing the geometry of the unipotent variety associated to a spets. From this, we determine the possible Springer correspondences, and we show that, as is true for algebraic groups, each special piece is rationally smooth, as is the full unipotent variety. © 2008 Birkhäuser Boston