LAUSR

We prove the following theorem. Let G be a finite group generated by unitary reflections in a complex Hermitian space $$V={\mathbb {C}}^\ell $$ V = C ℓ and let $$G'$$ G ′ be any reflection subgroup of G . Let $${\mathcal {H}}={\mathcal {H}}(G)$$ H = H ( G ) be the space of G -harmonic polynomials on V . There is a degree preserving isomorphism $$\mu :{\mathcal {H}}(G')\otimes {\mathcal {H}}(G)^{G'}\overset{\sim }{{\longrightarrow \;}}{\mathcal {H}}(G)$$ μ : H ( G ′ ) ⊗ H ( G ) G ′ ⟶ ∼ H ( G ) of graded $${\mathcal {N}}$$ N -modules, where $${\mathcal {N}}:=N_{{\text {GL}}(V)}(G)\cap N_{{\text {GL}}(V)}(G')$$ N : = N GL ( V ) ( G ) ∩ N GL ( V ) ( G ′ ) and $${\mathcal {H}}(G)^{G'}$$ H ( G ) G ′ is the space of $$G'$$ G ′ -fixed points of $${\mathcal {H}}(G)$$ H ( G ) . This generalises a result of Douglass and Dyer for parabolic subgroups of real reflection groups. An application is given to counting rational conjugates of reductive groups over $${\mathbb {F}}_q$$ F q .