2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/33953It is shown that almost all known solutions of the kind mentioned in the title are easily derived in a unified manner when a simple ansatz is imposed on the metric. The Whittaker solution is an exception, replaced by a new solution with the same equation of state.5 pages, revtexGeneral Relativity and Quantum Cosmologytext