PPN limit and cosmological gravitational waves as tools to constrain f(R)-gravity

We discuss the PPN Solar-System constraints and the GW stochastic background considering some recently proposed $f(R)$ gravity models which satisfy both cosmological and stability conditions. Using the definition of PPN-parameters $\gamma$ and $\beta$ in terms of $f(R)$-models and the definition of scalar GWs, we compare and discuss if it is possible to search for parameter ranges of $f(R)$-models working at Solar System and GW stochastic background scale.