Past exam of the mathematics course of the University of Cambridge / 2008 / ia / Paper 4 / 5D / c / Solution

To obtain a countable closure under real polynomial roots, start with $B_0=\{1,\pi \}$ and explicitly retain previous elements at each stage:Part (b) shows that $\varphi$ takes countable sets to countable sets; consequently all $B_n$ and then $X$ are countable by the countable union of countable sets theorem. Also $1,\pi \in X$.

For a nonzero polynomial $P(t)=c_0+\cdots +c_d{t}^d$ with all $c_j\in X$, choose stages $n_j$ containing the finitely many coefficients and put $N={\max }_j{n}_j$. The inclusions $B_n\subseteq B_{n+1}$ ensure that every coefficient belongs to $B_N$. Every real root of a polynomial $P$ therefore belongs to $\varphi (B_N)\subseteq B_{N+1}\subseteq X$. This countable set $X$ has all the required properties. Retaining $B_n$ explicitly avoids assuming the generally false inclusion $A\subseteq \varphi (A)$: for example, $\varphi (\{1\})$ does not contain $1$, since a nonzero polynomial all of whose coefficients are one is positive at $1$.