Are questions on numerical analysis on-topic?
I'm referring to this question
The OP hints that it might have some usefulness in DSP and embedded systems but did not give examples. (In other words, we were not told about the precision requirements and the computational resources.) However this is not my main point.
My main point is that some questions on DSP are asked because: (quoting myself)
Some DSP people might know a better way of doing this thing, ... (although this thing) ought to be solved by experts of another discipline
In this particular case, it is a numerical-methods question, which can be asked as-is on Stackoverflow (by showing a picture of a plot of the Lambert W function and asking how to approximate it), or Math.SE (by asking for several series expansions and iterative solutions, and evaluating their ease of computation, etc.)