NOTE: I know that countability of algebraic numbers has been proven on this site before, but I'm concerned about this specific hint they give and I don't know how to prove it using that advice.

I am doing an exercise in Rudin to prove that the set of algebraic numbers is countable. In particular,

A complex number is said to be algebraic if there are integers $a_0, \dots, a_n$, not all $0$, such that $a_0 z^n + a_1 z^{n-1} + \dots + a_n = 0$. Prove that the set of all algebraic numbers is countable.

Hint: For every positive integer $N$ there are only finitely many equations with $n + |a_0| + |a_1| + \dots + |a_n| = N$

I have proven the statement using essentially the method in this post, but I'm curious as to how one can use the hint to prove the statement.

  • 1
    $\begingroup$ Roger Cooke gives a solution to this problem and the rest in the book: minds.wisconsin.edu/handle/1793/67009 $\endgroup$ Jan 30 '15 at 16:57
  • $\begingroup$ @user26857 I usually go back at the end of every month and look through my posts and decide on an accept answer (or give a post a bounty if I think the best answer isn't good enough). I guess I just forgot. $\endgroup$
    – MCT
    Apr 1 '15 at 15:51

I believe that this hint helps you to establish that $\mathbb Z[X]$ is countable.

For a polynomial $f(X)=a_nX^n+\cdots +a_1X+a_0\in\mathbb Z[X]$, $a_n\neq 0$, define the number $h(f)$ by: $$h(f)= n+|a_n|+\cdots+|a_1|+|a_0|.$$ The function $h:\mathbb Z[X]\longrightarrow \mathbb N$ measures some kind of a complexity of a polynomial.

It is fairly obvious that the set $P_N=\{f(X)\in\mathbb Z[X]\mid h(f)= N\}$ is finite, and that $$\mathbb Z[X]= \bigcup_{N\geq 0}P_N.$$ Hence $\mathbb Z[X]$ is countable union of finite sets, hence countable. (Let me say, very nice trick!)

Now, the usual argument finishes the proof: the set of algebraic numbers is $$\bigcup_{f\in\mathbb Z[X]}\operatorname{roots}(f),$$ and it is countable union of finite sets, hence countable.

  • $\begingroup$ What exactly is $\mathbb{Z}[X]$? $\endgroup$ Mar 25 at 6:33
  • $\begingroup$ I think i understand now: $\mathbb{Z}[X]$ is the set of all finite complex polynomials with integer coefficients? $\endgroup$ Mar 25 at 19:56

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