Restricting invariants of unitary reflection groups

Abstract

Suppose that G is a finite, unitary reflection group acting on a complex vector space V and X is the fixed point subspace of an element of G. Define N to be the setwise stabilizer of X in G, Z to be the pointwise stabilizer, and C=N/Z. Then restriction defines a homomorphism from the algebra of G-invariant polynomial functions on V to the algebra of C-invariant functions on X. Extending earlier work by Douglass and Roehrle for Coxeter groups, we characterize when the restriction mapping is surjective for arbitrary unitary reflection groups G in terms of the exponents of G and C, and their reflection arrangements. A consequence of our main result is that the variety of G-orbits in the G-saturation of X is smooth if and only if it is normal.