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