I just asked this question: What approximation techniques exist for the square super-root function?, and there's some discussion in the comments (which I think is warranted) as to whether it's too purely mathematical to be a signal processing question. I offered a little explanation as to my reasoning in the comments, but thought I'd open the floor here since we're in the process of defining the range of "on-topic" questions for the site.
A more succinct explanation of my reasoning for the question [than I put in the comments] would be that it's about efficient implementation of one part of a nonlinear filter (e.g. in an image/audio effect or a control system). If it were geared to a coding question for a specific language, it might be better suited to stackoverflow (although I can see it getting kicked out of there for being too mathematical as well). I can also see it getting kicked out of math.SE for being too "applied". Most topics in signal processing often involve a mix of both programming and math, but is there a simple "test" we can come up with to determine when a question is too far in either direction?
At the moment, I really don't know of any SE site where questions that mix both math and programming have a true home (if I'm just missing something, let me know), and the solution of almost every problem that comes up in DSP involves some combination of both (e.g. the quintessential question(s) about the FFT).
By way of contrast, if a question about the efficient implementation of a special function isn't in scope for this site, would it be in scope if the question was about transforming some input into some output (e.g. "Efficient implementation of soft-clipping?"), and the answer ended up being the special function (e.g. some sigmoid function like erf(x))?