2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/116302Using construction of adiabatic vacuum states of a free scalar field on Robertson-Walker spacetime, using results of Luders and Roberts we prove that validity of the Hadamard condition implies smoothness of the scale factor.9 pages, no figuresMathematical Physics81T05, 81T20text