Continued fractions as dynamical systems

The continued fraction expansion of a real number may be studied by considering a suitable discrete dynamical system of dimension two. In the special case where the number to be expanded is a quadratic irrational, that is a positive irrational root of a polynomial of degree two, more insight may be gained by considering a new dynamical system of dimension three, where the state vector stores the coefficients of the quadratic polynomials resulting from the expansion process. We show that a number of constants of motions can be derived and exploited to explore the attracting set of the solutions. Links with the solution to Pell’s equations are also investigated.