On page 18 and 19 there is the term $\sigma_1...\sigma_n$. n is supposed to be any natural number including 0. My problem with this is that if n would be 0 you have the range $\sigma_1...\sigma_0$ which is not possible. On page 18 there is the same problem with i in the induction. I propose to change the start index of the sigma to 0, because a range of length 0 is still valid.
