Transfer matrix (5 min)

How is the definition of the transfer matrix $M$, defined by

$$\left( \begin{array}{c} a_1 \\ b_1 \end{array}\right)= \left... ...\left( \begin{array}{c} a_{N+1} \\ b_{N+1} \end{array}\right)?$$ (17)

Express $M$ as a product of matrices of the type (2.16).