I believe that question/answer of this form:
Q: Is there an online repository of basic identities/results about topic X?
A: Many of such identities can be found with the proofs at math.SE, I'll give here a list with links
is a valid question and a valid answer for this site.
We already had some questions of this type, e.g. this one: A comprehensive list of binomial identities?
I've tried to make such a post for basics of cardinal arithmetics here, but I can imagine such post being useful for other topics, too: combinatorial identities or perhaps identities involving binomial coefficients (we have plenty of more-or-less duplicate questions asking about a proof of Chu-Vandermonde identity here), trigonometric identities, basic limits, sums of series, basic facts about image and preimage, basic facts about injective/surjective/bijective functions, etc. (Probably many of mathematical topics that have "list article" at Wikipedia would qualify.)
Nevertheless, I've cheated a little, because I've tried to use this questions and answer to "organize" posts at this site a little - in this way we (hopefully) get list of questions/answers relevant to given topic. (So my main intention was not to ask a question and get an answer, but to make an "organizer" post - I was not able to think of better name for this type of post.)
I think that post like this might be useful mainly for the reasons that:
It helps when searching for duplicates. (It's probable that such a basic question has already been answered here, but it's not always easy to find it.)
It helps askers to avoid adding duplicate question.
A few more minor notes.
- Obviously this would make a post more suitable for wiki-format then Q&A-format but Stack Overflow has elements of wiki in its hybrid design, says this blog post.
I do not suggest that we should start right now massively doing this, but I think this is worth considering for topics, where duplicates of some basic questions appear very frequently. (Once an "organizer" post exists, it is not too much work to add links to it, whenever a new duplicate question, which is not covered in the post, appears.)
I am not doing this hunting for points (although in my post about cardinal arithmetic, I link to some questions I've answered). I hope you'll take my word for this.
Two questions, which lead me to thinking about posting something like this were How do you search for duplicates and a very recent Call for discussion: Compiling an introductory resource for (say) Abstract Algebra, from MSE answers. - which has a somewhat similar flavor to this question of mine.
I'd be glad to hear the opinion of other users, whether such format is good for this site and whether they think such posts might be useful.