2016-3.md

course	course_year	question_number	tags	title	year
Analysis II	IB	3	IB 2016 Analysis II	Paper 1, Section II, G	2016

Let $(X, d)$ be a metric space.

(a) What does it mean to say that $\left(x_{n}\right){n}$ is a Cauchy sequence in $X$ ? Show that if $\left(x{n}\right)_{n}$ is a Cauchy sequence, then it converges if it contains a convergent subsequence.

(b) Let $\left(x_{n}\right)_{n}$ be a Cauchy sequence in $X$.

(i) Show that for every $m \geqslant 1$, the sequence $\left(d\left(x_{m}, x_{n}\right)\right){n}$ converges to some $d{m} \in \mathbb{R}$.

(ii) Show that $d_{m} \rightarrow 0$ as $m \rightarrow \infty$.

(iii) Let $\left(y_{n}\right){n}$ be a subsequence of $\left(x{n}\right){n}$. If $\ell, m$ are such that $y{\ell}=x_{m}$, show that $d\left(y_{\ell}, y_{n}\right) \rightarrow d_{m}$ as $n \rightarrow \infty$.

(iv) Show also that for every $m$ and $n$,

$$d_{m}-d_{n} \leqslant d\left(x_{m}, x_{n}\right) \leqslant d_{m}+d_{n}$$

(v) Deduce that $\left(x_{n}\right){n}$ has a subsequence $\left(y{n}\right)_{n}$ such that for every $m$ and $n$,

$$d\left(y_{m+1}, y_{m}\right) \leqslant \frac{1}{3} d\left(y_{m}, y_{m-1}\right)$$

and

$$d\left(y_{m+1}, y_{n+1}\right) \leqslant \frac{1}{2} d\left(y_{m}, y_{n}\right)$$

(c) Suppose that every closed subset $Y$ of $X$ has the property that every contraction mapping $Y \rightarrow Y$ has a fixed point. Prove that $X$ is complete.