Abstract
Let \((X,d,\mu)\) be a set with quasimetric \(d\) and \(\sigma\) -finite measure \(\mu\) , and let \(I=(0,t_0)\) , \(0<t_0\le\infty\) . We consider the classes \(\mathcal H^{p,r}\) , \(0<p,r\le\infty\) , consisting of complex-valued measurable functions \(u\) on \(X\times I\) for which the maximal function \(\mathcal N u(x):=\sup\{|u(y,t)|\colon d(x,y)<t\}\) , \(x\in X\) , 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 \(\mathcal H^{p,r}\) , we consider generalizations of the classical Hardy–Littlewood inequalities as well as fractional integration operators.