On the Wikipedia page on Arithmetic Functions, the section Relations Among The Functions makes frequent references to a variable $\delta$ (or is it a function? Some other kind of value?).
It's probably just a random choice of variable; but it seems an oddly specific symbol to suddenly pull out of nowhere.
Nowhere on the page does it explain what $\delta$ means in this context, which renders all those lovely lines of formulae meaningless. If $\delta$ has some special meaning that article's author either (a) thought everyone would simply know or (b) if they didn't know then they shouldn't be reading the article anyway, then... Well, the article shouldn't be on Wikipedia!
Can anyone enlighten me? I probably missed something obvious, in which case I'd appreciate someone explaining my error.
That $\delta$ is a dummy variable in each summation over divisors. For example, in the equation $$ \sum_{\delta | 10} f(\delta) = f(1) + f(2) + f(5) + f(10) $$ the expansion on the right shows you that there really is no "$\delta$".
As @MPW comments, the choice of $\delta$ is meant to suggest divisor. I'd have used $d$. You could even edit the wikipedia page to make that change.