Power-laws f(R) theories are cosmologically unacceptable

In a recent paper, 1 we have shown that f(R) = R + mu R-n modified gravity dark energy models are not cosmologically viable because during the matter era that precedes the accelerated stage the cosmic expansion is given by a similar to t(1/2) rather than a similar to t(2/3), where a is a scale factor and t is the cosmic time. A recent work by Capozziello et al.(2) criticized our results presenting some apparent counter-examples to our claim in f(R) = mu R-n models. We show here that those particular R-n models can produce an expansion as a similar to t(2/3) but this does not connect to a late-time acceleration. Hence, though acceptable f(R) dark energy models might exist, the R-n models presented in Capozziello et al. are not cosmologically viable, confirming our previous results in Ref. 1.