`  ::First of all I find it disturbing that the ``is an element of`` symbol wasn't showing up for JKeck, since there seems to be nothing irregular about the way it was entered (looks like it came from the ``Math and logic`` collection in the roll-down menu from the edit pages, which gives: ∈). If you have this problems you are probably missing loads of symbols in math articles, since this is the privileged way to represent them. You could try to replace the symbol by ∈ giving ∈ (or by clicking on the symbol in Math and logic yourself, which might resolve some coding issue) to see it that works better. TeX code in WP produces images, which avoids such problems but looks bad in in-line formulas (however much the formulas themselves are of superior quality). If this is really a problem of your local installation, the solution cannot be to change the WP articles.  ::As for problems understanding, I think you really don't need to understand the axiom of choice unless you are a mathematician interested in foundations. In physics you will never come about a situation where a result depends on the axiom of choice, because it talks about provability in set theory, only. Probably the opening sentence should make this clear. For any reasonable collection of non-empty sets one could encounter in real life, a choice function is easily found without use of axioms, and this is the reason that the axiom seems reasonable, but in mathematics it gets applied in highly abstract and complicated situations, where the ``real world`` justification for the existence of choice functions no longer suffices.`