Radial function explained

In mathematics, a radial function is a real-valued function defined on a Euclidean space Rⁿ whose value at each point depends only on the distance between that point and the origin. The distance is usually the Euclidean distance. For example, a radial function Φ in two dimensions has the form^[1]

\Phi(x,y)=\varphi(r), r=\sqrt{x^2+y^2}

where φ is a function of a single non-negative real variable. Radial functions are contrasted with spherical functions, and any descent function (e.g., continuous and rapidly decreasing) on Euclidean space can be decomposed into a series consisting of radial and spherical parts: the solid spherical harmonic expansion.

A function is radial if and only if it is invariant under all rotations leaving the origin fixed. That is, ƒ is radial if and only if

f\circ\rho=f

for all, the special orthogonal group in n dimensions. This characterization of radial functions makes it possible also to define radial distributions. These are distributions S on Rⁿ such that

S[\varphi]=S[\varphi\circ\rho]

for every test function φ and rotation ρ.

Given any (locally integrable) function ƒ, its radial part is given by averaging over spheres centered at the origin. To wit,

\phi(x)=

	1
	\omega_n-1

\int
	S^n-1

f(rx')dx'

where ω_n-1 is the surface area of the (n-1)-sphere S^n-1, and, . It follows essentially by Fubini's theorem that a locally integrable function has a well-defined radial part at almost every r.

The Fourier transform of a radial function is also radial, and so radial functions play a vital role in Fourier analysis. Furthermore, the Fourier transform of a radial function typically has stronger decay behavior at infinity than non-radial functions: for radial functions bounded in a neighborhood of the origin, the Fourier transform decays faster than R^-(n-1)/2. The Bessel functions are a special class of radial function that arise naturally in Fourier analysis as the radial eigenfunctions of the Laplacian; as such they appear naturally as the radial portion of the Fourier transform.

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News: 2022-03-17 . Radial Basis Function - Machine Learning Concepts . en-US . Machine Learning Concepts - . 2022-12-23.

Radial function explained

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