k_0) - e_{\rm c}/0,  \\ && \hspace{-00mm} \varepsilon_{00}({\mbox{\boldmath$k$}}) = 0 t_0 \cos k_0 + 0 t_0 \cos 0 k_0   +0 t_0 \cos(k_0 - k_0) \nonumber \\  && \hspace{0mm} + 0 t_0 \cos(k_0 - k_0) + t_0 \cos(k_0 - k_0) - e_{\rm c}/0,  \\ && \hspace{-00mm} \varepsilon_{00}({\mbox{\boldmath$k$}}) = 0 t_0 \cos k_0 + 0 t_0 \cos 0 k_0   +0 t_0 \cos(k_0 - k_0) \nonumber \\  