On [page 168](http://books.google.com/books?id=M6DvzoKlcicC&lpg=PP1&dq=mathematical%2520fallacies%2520and%2520paradoxes&pg=PA168#v=onepage&q&f=false) of *Mathematical Fallacies and Paradoxes*, it states that the fact that the series 


$1 - \frac{1}{2} + \frac{1}{3} - \frac{1}{4} + \cdots $


has a sum depends on the Axiom of Choice. Where does the AC come in to play? I know that if the terms are permuted, we can get any sum we want, and I can see how the AC might be involved there, but just the fact that $1 - \frac{1}{2} + \frac{1}{3} - \frac{1}{4} + \cdots $ converges?



Why does this sum depend on the Axiom of Choice?
It doesn't seem to me that you need any choice principle at all to prove that this series converges. The Alternating Series Test that appears in any elementary calculus books seems to do the job, and doesn't seem to require any amount of AC. If $\Sigma\_n (-1)^n a\_n$ is an alternating series, with $a\_n$ descending to $0$, then the finite partial sums up to positive term are descending and the partial sums up to a negative term are increasing, and that these two sequences converge to the same limit. 


So I'm not sure which theorem had been "called upon to show" that the series converges, but unless I am mistaken, it must have been something other than what our students would call the Alternating Series Test.




---


Edit. More generally, I claim that the convergence of a series can never depend on the Axiom of Choice. Suppose that $r=\langle a\_0,a\_1,\ldots\rangle$ is a sequence of real numbers, and we are consider the series $\Sigma a\_n$. The assertion "$\Sigma a\_n$ converges" is a statement about $r$ having complexity $\Sigma^1\_1(r)$, that is, an analytic fact about $r$, and therefore has the same truth value in the set theoretic universe $V$ as it has in the relativized constructible universe $L[r]$, where AC holds. In particular, the series converges in $V$ if and only if it converges in $L[r]$. 


Conclusion. If you have a series $\Sigma a\_n$ defined in a sufficiently concrete manner and you can prove in ZFC that it converges, then you can prove in ZF that it converges.


(The technical requirement here about sufficiently concrete is that the description of $r=\langle a\_0,a\_1,\ldots\rangle$ should be absolute from $V$ to $L[r]$. This would be true of any arithmetically definable series as in the question, defined by an arithmetic formula, or even a Borel definition or a $\Sigma^1\_2$ definition.)


