The cushion surface is the quartic surface resembling a squashed round cushion on a barroom stool and given by the equation
Cushion Surface
See also
Chair Surface, Paper Bag Surface, Quartic SurfaceExplore with Wolfram|Alpha
References
Nordstrand, T. "Surfaces." http://jalape.no/math/surfaces.Cite this as:
Weisstein, Eric W. "Cushion Surface." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/CushionSurface.html