}{{{\text{d}}\tau }} = \sigma (z) - y \hfill \\ \frac{{{\text{d}}z}}{{{\text{d}}\tau }} = \mu \left( {\frac{{0 + \beta_{0} R_{0} }}{{0 + \beta_{0} \overline{f} (\tau )R_{0} \sigma^{0} (z)}}y - \sigma (z)z} \right) \hfill \\ \end{gathered} \right.. $$\end{document}$$  