In this paper we study the problem of the membership of H φ in the Hilbert-Schmidt class, when φ ∈ L ∞ (Ω) and Ω is a planar domain. We find a necessary and sufficient condition. We apply this result to the problem of joint membership of Hϕ and Hϕ in the Hilbert-Schmidt class. Using the notion of Berezin Transform and a result of K. Zhu we are able to give a necessary and sufficient condition. Finally, we recover a result of Arazy, Fisher and Peetre on the case Hϕ with ϕ holomorphic.
Mathematics Subject Classification (2000). Primary 47B35; Secondary 47B38.