This is an announcement for the paper "Optimal domain for the Hardy operator" by Olvido Delgado and Javier Soria. Abstract: We 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 $\delta$-ring. A precise description is given for the case of the minimal Lorentz spaces. Archive classification: Functional Analysis; Classical Analysis and ODEs Mathematics Subject Classification: 46E30, 46B25 Remarks: 15 pages The source file(s), DeSo.tex: 40756 bytes, is(are) stored in gzipped form as 0602426.gz with size 13kb. The corresponding postcript file has gzipped size 66kb. Submitted from: soria@ub.edu The paper may be downloaded from the archive by web browser from URL http://front.math.ucdavis.edu/math.FA/0602426 or http://arXiv.org/abs/math.FA/0602426 or by email in unzipped form by transmitting an empty message with subject line uget 0602426 or in gzipped form by using subject line get 0602426 to: math@arXiv.org.