A quartic surface whose graphical representation looks like an indented bar stool cushion. The equation for the surface is as follows:

z2x2 - z4 - 2zx2 + 2z3 + x2 - z2 - (x2 - z)2 - y4 - 2x2y2 - y2z2 + 2y2z + y2 = 0