+ $-\omega^2M_1u\exp(i(ska-\omega t))=
+ C\exp(-i\omega t)[v\exp(iska)+v\exp(i(s-1)ka)-2u\exp(iska)]$\\
+ $\Rightarrow -\omega^2M_1u=Cv(1+\exp(-ika))-2Cu$\\
+ $-\omega^2M_2v\exp(i(ska-\omega t))=
+ C\exp(-i\omega t)[u\exp(i(s+1)ka)+u\exp(iska)-2v\exp(iska)]$\\
+ $\Rightarrow -\omega^2M_2v=Cu[\exp(ika)+1]-2Cv$\\