The double sphere is the degenerate quartic surface
obtained by squaring the left-hand side of the equation of a usual sphere
The double sphere is the degenerate quartic surface
obtained by squaring the left-hand side of the equation of a usual sphere
Weisstein, Eric W. "Double Sphere." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DoubleSphere.html