Cosmological constraints on $\gamma$-gravity models
  In this paper we place observational constraints on the well-known
$\gamma$-gravity $f(R)$ model using the latest cosmological data, namely we use
the latest growth rate, Cosmic Microwave Background, Baryon Acoustic
Oscillations, Supernovae type Ia and Hubble parameter data. Performing a joint
likelihood analysis we find that the $\gamma$-gravity model is in very good
agreement with observations. Utilizing the AIC statistical test we
statistically compare the current $f(R)$ model with $\Lambda$CDM cosmology and
find that they are statistically equivalent. Therefore, $\gamma$-gravity can be
seen as a useful scenario toward testing deviations from General Relativity.
Finally, we note that we find somewhat higher values for the $f(R)$ best-fit
values compared to those mentioned in the past in the literature and we argue
that this could potential alleviate the halo-mass function problem.
