3.II.12F

Topics in Analysis
Part II, 2007

(i) State and prove Liouville's theorem on approximation of algebraic numbers by rationals.

(ii) Consider the continued fraction

x=1a1+1a2+1a3+1a4+x=\frac{1}{a_{1}+\frac{1}{a_{2}+\frac{1}{a_{3}+\frac{1}{a_{4}+\ldots}}}}

where the aja_{j} are strictly positive integers. You may assume the following algebraic facts about the nnth convergent pn/qnp_{n} / q_{n}.

pnqn1pn1qn=(1)n,qn=anqn1+qn2.p_{n} q_{n-1}-p_{n-1} q_{n}=(-1)^{n}, \quad q_{n}=a_{n} q_{n-1}+q_{n-2} .

Show that

pnqnx1qnqn+1\left|\frac{p_{n}}{q_{n}}-x\right| \leqslant \frac{1}{q_{n} q_{n+1}}

Give explicit values for ana_{n} so that xx is transcendental and prove that you have done SO.