** Determinant of $M$ (10 min)

Consider the case $k_1=k_{N+1}$ in (2.14). Use the definitions for $T^1$ ($T^n$ correspondingly) and $M$

$$T^1=\frac{1}{2k_1}\left( \begin{array}{cc} (k_1+k_2)e^{i(k_2-k_1)... ..._1)x_1} & (k_1+k_2)e^{-i(k_2-k_1)x_1} \end{array} \right),\quad M=T^1T^2...T^N,$$ (20)

to show that the determinant of the transfer matrix $\det(M)=1$.