Wiktionary
n. (context mathematics English) A set which is unchanged by a function or other mapping. Formally: a set, S, is a fixed set of a function, f, if and only if for all x in S, f(x)=x.
Usage examples of "fixed set".
The shaft of sunlight would drift visibly in the course of the interview, but at the editing stage everything could be stripped back to reflectance values, and recomputed with a fixed set of rather more flattering light sources.
It would work on a ship, on a building, on a tank, a truck-anything that has a fixed set of radar-reflective properties that the computer can be programmed to nullify.
Now the question occurred: How did Tom know that all those signs were part of a fixed set?
People were apt to assume that a given game had an eternally fixed set of rules, when in fact there were endless variations.
Moreover, our ability to explain a vast number of astrophysical observations of far-flung regions of the cosmos using one, fixed set of physical principles leads us to believe that the same laws do hold true everywhere.
Animals have to rely on a fixed set of signals for communication purposesor else on varying the strength of the same signal.