We have the relations

p_n = a_{n}p_{n-1} + b_{n}p_{n-2}, q_n = a_{n}q_{n-1} + b_{n}q_{n-2}.

b2 b3 b4

In the case of the fraction a1 - -- -- -- ..., we have the

a2 - a3 - a4 -

relations

p_n = a_{n}p_{n-1} - b_{n}p_{n-2}, q_n= a_{n}q_{n-1} - b_{n}q_{n-2}.