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Power-law cosmic expansion in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>f</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mi>R</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math>gravity models
We show that within the class of $f(R)$ gravity theories, Friedmann-Lema\^{\i}tre-Robertson-Walker 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.