function in Hp and a function in Hq. For example: every function in H1 is the product of two functions in H2; every function in Hp, p < 1, can be expressed as product of several functions in some Hq, q > 1. Real-variable techniques on the unit circle Real-variable techniques, mainly associated to the study of real Hardy spaces defined on Rn (see below), are also used in the simpler framework of the circle. It is a common practice to allow for complex functions (or distributions) in these "real" spaces. The definition that follows does not distinguish between real or complex case. Let Pr denote the Poisson kernel on the unit circle T. For a distribution f on the unit circle, set where the star indicates convolution between the distribution f and the function eiθ → Pr(θ) on the circle. Namely, (f ∗ Pr)(eiθ) is the result of the action of f on the C∞-function defined on the unit circle by For 0 < p < ∞, the real Hardy space Hp(T) consists of distributions f such that M f is in Lp(T). The function F defined on the unit disk by F(reiθ) = (f ∗ Pr)(eiθ) is harmonic, and M f is the radial maximal function of F. When M f belongs to Lp(T) and p ≥ 1, the distribution f "is" a function in Lp(T), namely the boundary value of F. For p ≥ 1, the real Hardy space Hp(T) is a subset of Lp(T). Conjugate function To every real trigonometric polynomial u on the unit circle, one associates the real conjugate polynomial v such that u + iv extends to a holomorphic function in the unit disk, This mapping u → v extends to a bounded linear operator H on Lp(T), when 1 < p < ∞ (up to a scalar multiple, it is the Hilbert transform on the unit circle), and H also maps L1(T) to weak-L1(T). When 1 ≤ p < ∞, the following are equivalent for a real valued integrable function f on the unit circle: the function f is the real part of some function g ∈ Hp(T) the function f and its conjugate H(f) belong to Lp(T) the radial maximal function M f belongs to Lp(T). When 1 < p < ∞, H(f) belongs to Lp(T) when f ∈ Lp(T), hence the real Hardy space Hp(T) coincides with Lp(T) in this