Abstract. We consider certain Littlewood-Paley operators and prove characterization of some function spaces in terms of those operators. When treating weighted Lebesgue spaces, a generalization to weighted spaces will be made for Hörmander's theorem on the invertibility of homogeneous Fourier multipliers. Also, applications to the theory of Sobolev spaces will be given.
IntroductionWe consider the Littlewood-Paley function on R n defined by, where ψ t (x) = t −n ψ(t −1 x). The following result of Benedek, Calderón and Panzone [2] on the L p boundedness, 1 < p < ∞, of g ψ is well-known.Theorem A. We assume (1.1) for ψ andwhereBy the Plancherel theorem, it follows that g ψ is bounded on L 2 (R n ) if and only if m ∈ L ∞ (R n ), where m(ξ) = ∞ 0 |ψ(tξ)| 2 dt/t, which is a homogeneous function of degree 0. Here the Fourier transform is defined aŝ ψ(ξ) = R n ψ(x)e −2πi x,ξ dx, x, ξ = n k=1x k ξ k .