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Supersphere


Supersphere

The supersphere is the algebraic surface that is the special case of the superellipse with a=b=c. It has equation

 |x/a|^n+|y/a|^n+|z/a|^n=1
(1)

or

 |x|^n+|y|^n+|z|^n=a^n
(2)

for radius a and exponent n.

Special cases are summarized in the following table, together with their volumes.

nsurfacevolume with a=1
1regular octahedron4/3
2sphere(4pi)/3
4([Gamma(1/4)]^4)/(6sqrt(2)pi)
6Hauser's "cube"(2[Gamma(1/6)]^3)/(27sqrt(pi))
inftycube8

The surface area is given by

 S_n=48int_0^(pi/4)int_0^(sec^(-1)(sqrt(2+tan^2phi)))sqrt((cos^mthetasin^2theta+sin^(2n)theta(cos^mphi+sin^mphi))/([cos^ntheta+sin^ntheta(cos^nphi+sin^nphi)^(n+1/2)]))dthetadphi
(3)

(Trott 2006, p. 301), where m=2n-2.

The volume enclosed is given by

V_n=8a^3int_0^1int_0^((1-x^n)^(1/n))(1-x^n-y^n)^(1/n)dydx
(4)
=(8[Gamma(1+1/n)]^3)/(Gamma(1+3/n))a^3.
(5)

As n->infty, the solid becomes a cube, so

 lim_(n->infty)V_n=8a^3
(6)

as it must. This is a special case of the integral 3.2.2.2

 intintint_(x>=0,y>=0,z>=0; (x/a)^p+(y/b)^q+(z/c)^r<=1)x^(alpha-1)y^(beta-1)z^(gamma-1)dxdydz=(a^alphab^betac^gamma)/(pqr)(Gamma(alpha/p)Gamma(beta/q)Gamma(gamma/r))/(Gamma(alpha/p+beta/q+gamma/r+1))
(7)

in Prudnikov et al. (1986, p. 583). The cases n=2 and n=6 appear to be the only integers whose corresponding solids have simple moment of inertia tensors, given by

I_2=[2/5Ma^2 0 0; 0 2/5Ma^2 0; 0 0 2/5Ma^2]
(8)
I_6=[3/5Ma^2 0 0; 0 3/5Ma^2 0; 0 0 3/5Ma^2].
(9)

See also

Sphere, Superegg, Superellipsoid

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References

Gray, A. Modern Differential Geometry of Curves and Surfaces with Mathematica, 2nd ed. Boca Raton, FL: CRC Press, p. 292, 1997.Hauser, H. "Gallery of Singular Algebraic Surfaces: Cube." https://homepage.univie.ac.at/herwig.hauser/gallery.html.POV-Ray Team. "Superquadratic Ellipsoid." §4.5.1.10 in Persistence of Vision Ray-Tracer Version 3.1g User's Documentation, p. 199, May 1999.Prudnikov, A. P.; Brychkov, Yu. A.; and Marichev, O. I. Integrals and Series, Vol. 1: Elementary Functions. New York: Gordon and Breach, 1986.Trott, M. The Mathematica GuideBook for Numerics. New York: Springer-Verlag, pp. 301-303, 2006. http://www.mathematicaguidebooks.org/.

Cite this as:

Weisstein, Eric W. "Supersphere." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Supersphere.html

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