We show that within the class of f(R) gravity theories, FLRW power-law perfect fluid solutions only exist for R^n gravity. This significantly restricts the set of exact cosmological solutions which have similar properties to what is found in standard General Relativity.
@article{arxiv.0906.3860,
title = {Power-law cosmic expansion in f(R) gravity models},
author = {Naureen Goheer and Julien Larena and Peter K. S. Dunsby},
journal= {arXiv preprint arXiv:0906.3860},
year = {2009}
}