Let (X, d, ) be a set with quasimetric d and -finite measure, and let I= (0, t₀), 0<t₀. We consider the classes H^p, r, 0<p, r, consisting of complex-valued measurable functions u on X I for which the maximal function N u (x): =\|u (y, t) | d (x, y) <t\, x X, belongs to the Lorentz space L^p, r (X). In special cases, these classes are extensions of certain Hardy and Hardy–Lorentz spaces (of analytic, harmonic, etc. , functions). For functions in the classes H^p, r, we consider generalizations of the classical Hardy–Littlewood inequalities as well as fractional integration operators.
Krotov et al. (Mon,) studied this question.