How to specify +/- in a function definition.

[AlexFekken]

Since the problem is apparently caused by the domain of the functions ending abruptly, you can (arguably) get a "better" graph by extending the domain, e.g. making the functions zero where they are now not defined:

FZn= sqrt(max(0,16-X^2-Y^2)) // (sorry about the edits)

Increasing the grid size will also give a somewhat better picture (with or without domain extension) but unfortunately it is limited to 100x100. Now *that* limitation should be easy to change by the developers.