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3 pagesIn the recent paper astro-ph/0603703 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 $a ~ t^{1/2}$ rather than $t^{2/3}$. In this short note we wish to comment on the recent paper astro-ph/0604431 which criticised our results. We show here that the $R^n$ models presented in astro-ph/0604431 cannot generate a stage with $a ~ t^{2/3}$ preceding a stage of accelerated expansion. Hence, though acceptable $f(R)$ dark energy models might exist, the $R^n$ models presented in astro-ph/0604431 are not viable, confirming our previous results

POWER-LAWS f(R) THEORIES ARE COSMOLOGICALLY UNACCEPTABLE