2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/176035The Dolgov-Kawasaki instability discovered in the matter sector of the modified gravity scenario incorporating a 1/R correction to Einstein gravity is studied in general f(R) theories. A stability condition is found in the metric version of these theories to help ruling out models that are unviable from the theoretical point of view.4 pages, revtex, to appear in Phys. Rev. D. In the revised version, an error concerning the Palatini version of these theories has been corrected and the references updatedAstrophysicsGeneral Relativity and Quantum Cosmologytext