How to specify +/- in a function definition.

[webmasterpdx]

That does make it look better, but it's not really plotting a sphere there. I think the problem is actually a bug. The plot should be flush with the plane z=0. It's not just rounding error, as if I plot sqrt(100-(x^2+y^2)), the blank areas are higher than 1 off the plane. That looks like a bug.