Consider the question Big O exponential: proving NP is a subset
Asymptotic notation questions have been pretty standard for Stack Overflow. This fairly interesting beginner question is up, but has already accumulated a couple of 'close' votes. One of them is for 'off topic' but, possibly because I'm a theory guy, it seems very much on topic for programming and related topics.
You could argue for "computer science" potentially, but it would seem this, like questions on, oh, NP-complete problems and such, are things serious professional programmers should know and are interested in.
You could also make a case for mathematics and event the theory stack, although it's probably a little light for theory. But with all of these separate categories, there seems to me to be a real risk that interesting questions will be modded/closed/moved until the OP loses interest.
Perhaps we've gotten a little too compartmentalized?
[ubuntu]
. You can have 5 tags per question. I haven't heard of anyone running out of space for tags yet. I don't think this is a real problem. Tags don't need to cover all of the specifics, that's what the question body is for. And the reason we don't have one book encyclopediae is because the entries don't all fit in a single book. That's not a problem with websites...