   \delta\left(     \left|        \varepsilon_{\bf k} - \varepsilon_{({\bf k}+{\bf q})}     \right|     + \omega_{\bf q} - \lambda   \right)    \frac{     \left| g_{\bf q} \right|^{0}     \Theta\left[       \omega_{\bf q} -        \left| \varepsilon_{\bf k} - \varepsilon_{({\bf k}+{\bf q})} \right|     \right]   }{     \left|       \varepsilon_{\bf k} - \varepsilon_{({\bf k}+{\bf q})}     \right| 