Paper 4, Section II, E

Analysis II | Part IB, 2019

(a) (i) Show that a compact metric space must be complete.

(ii) If a metric space is complete and bounded, must it be compact? Give a proof or counterexample.

(b) A metric space (X,d)(X, d) is said to be totally bounded if for all ϵ>0\epsilon>0, there exists NNN \in \mathbb{N} and {x1,,xN}X\left\{x_{1}, \ldots, x_{N}\right\} \subset X such that X=i=1NBϵ(xi).X=\bigcup_{i=1}^{N} B_{\epsilon}\left(x_{i}\right) .

(i) Show that a compact metric space is totally bounded.

(ii) Show that a complete, totally bounded metric space is compact.

[Hint: If (xn)\left(x_{n}\right) is Cauchy, then there is a subsequence (xnj)\left(x_{n_{j}}\right) such that

jd(xnj+1,xnj)<.]\left.\sum_{j} d\left(x_{n_{j+1}}, x_{n_{j}}\right)<\infty .\right]

(iii) Consider the space C[0,1]C[0,1] of continuous functions f:[0,1]Rf:[0,1] \rightarrow \mathbb{R}, with the metric

d(f,g)=min{01f(t)g(t)dt,1}.d(f, g)=\min \left\{\int_{0}^{1}|f(t)-g(t)| d t, 1\right\} .

Is this space compact? Justify your answer.

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