Cyclotomic unit

This article is about a type of transform used in classical potential theory, a topic in mathematics.

The Kelvin transform is a device used in classical potential theory to extend the concept of a harmonic function, by allowing the definition of a function which is 'harmonic at infinity'. This technique is also used in the study of subharmonic and superharmonic functions.

In order to define the Kelvin transform f^* of a function f, it is necessary to first consider the concept of inversion in a sphere in Rⁿ as follows.

It is possible to use inversion in any sphere, but the ideas are clearest when considering a sphere with centre at the origin.

Given a fixed sphere S(0,R) with centre 0 and radius R, the inversion of a point x in Rⁿ is defined to be

$x^{*}=\frac{R^2}{|x{|}^2}x.$

A useful effect of this inversion is that the origin 0 is the image of $\mathrm{\infty }$, and $\mathrm{\infty }$ is the image of 0. Under this inversion, spheres are transformed into spheres, and the exterior of a sphere is transformed to the interior, and vice versa.

The Kelvin transform of a function is then defined by:

If D is an open subset of Rⁿ which does not contain 0, then for any function f defined on D, the Kelvin transform f^* of f with respect to the sphere S(0,R) is

$f^{*}(x^{*})=\frac{|x{|}^{n-2}}{R^{2n-4}}f(x)=\frac{1}{|x^{*}{|}^{n-2}}f(x)=\frac{1}{|x^{*}{|}^{n-2}}f\left(\frac{R^2}{|x^{*}{|}^2}{x}^{*}\right).$

One of the important properties of the Kelvin transform, and the main reason behind its creation, is the following result:

Let D be an open subset in Rⁿ which does not contain the origin 0. Then a function u is harmonic, subharmonic or superharmonic in D if and only if the Kelvin transform u^* with respect to the sphere S(0,R) is harmonic, subharmonic or superharmonic in D^*.

This follows from the formula

$\mathrm{\Delta }{u}^{*}(x^{*})=\frac{R^4}{|x^{*}{|}^{n+2}}(\mathrm{\Delta }u)\left(\frac{R^2}{|x^{*}{|}^2}{x}^{*}\right).$

See also

• William Thomson, 1st Baron Kelvin
• Inversive geometry

References

• 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
  
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• 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
  
  My blog: http://www.primaboinca.com/view_profile.php?userid=5889534

• 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
  
  My blog: http://www.primaboinca.com/view_profile.php?userid=5889534