\begin{equation} y=\left\{\begin{array}{c} y_{\rm break}(\frac{x}{x_{\rm break}})^{\gamma_1} (x<{x_{\rm break}})\\ y_{\rm break}(\frac{x}{x_{\rm break}})^{\gamma_2} (x>={x_{\rm break}}) \end{array} \right .\notag \end{equation}