  It has been proposed that cosmic acceleration or inflation can be driven by
replacing the Einstein-Hilbert action of general relativity with a function
f(R) of the Ricci scalar R. Such f(R) gravity theories have been shown to be
equivalent to scalar-tensor theories of gravity that are incompatible with
Solar System tests of general relativity, as long as the scalar field
propagates over Solar System scales. Specifically, the PPN parameter in the
equivalent scalar-tensor theory is gamma=1/2, which is far outside the range
allowed by observations. In response to a flurry of papers that questioned the
equivalence of f(R) theory to scalar-tensor theories, it was recently shown
explicitly, without resorting to the scalar-tensor equivalence, that the vacuum
field equations for 1/R gravity around a spherically symmetric mass also yield
gamma= 1/2. Here we generalize this analysis to f(R) gravity and enumerate the
conditions that, when satisfied by the function f(R), lead to the prediction
that gamma=1/2.
