Diagnostic of $f(R)$ under the $Om(z)$ function
  We perform the two$-$point diagnostic for the $Om(z)$ function proposed by
Sahni ${\it et al}$ in 2014 for the Starobinsky and Hu & Sawicki models in
$f(R)$ gravity. We show that the observed values of the $Omh^2$ function can be
explained in $f(R)$ models while in LCDM the $Omh^2$ funticon is expected to be
a redshift independent number. We perform the analysis for some particular
values of $\Omega_m^0$ founding a cumulative probability ($P(\chi^2 \leq
\chi^2_{{\it model}})$) $P \sim 0.16$ or $\sim0.09$ for the better cases versus
a cumulative probability of $P \sim 0.98$ in the $\Lambda$CDM scenario. We also
show that these models present a characteristic signature around the interval
between $z\sim 2$ and $z\sim 4$, that could be confronted with future
observations using the same test.
