2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/109034We study the optimal domain for the Hardy operator considered with values in a rearrangement invariant space. In particular, this domain can be represented as the space of integrable functions with respect to a vector measure defined on a $δ$-ring. A precise description is given for the case of the minimal Lorentz spaces.15 pagesFunctional AnalysisClassical Analysis and ODEs46E30, 46B25text