# Paper 4, Section II, D

(a) Define what it means for a set to be countable.

(b) Let $A$ be an infinite subset of the set of natural numbers $\mathbb{N}=\{0,1,2, \ldots\}$. Prove that there is a bijection $f: \mathbb{N} \rightarrow A$.

(c) Let $A_{n}$ be the set of natural numbers whose decimal representation ends with exactly $n-1$ zeros. For example, $71 \in A_{1}, 70 \in A_{2}$ and $15000 \in A_{4}$. By applying the result of part (b) with $A=A_{n}$, construct a bijection $g: \mathbb{N} \times \mathbb{N} \rightarrow \mathbb{N}$. Deduce that the set of rationals is countable.

(d) Let $A$ be an infinite set of positive real numbers. If every sequence $\left(a_{j}\right)_{j=1}^{\infty}$ of distinct elements with $a_{j} \in A$ for each $j$ has the property that

$\lim _{N \rightarrow \infty} \frac{1}{N} \sum_{j=1}^{N} a_{j}=0$

prove that $A$ is countable.

[You may assume without proof that a countable union of countable sets is countable.]

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