# Riesz potential

In mathematics, the Riesz potential is a potential named after its discoverer, the Hungarian mathematician Marcel Riesz. In a sense, the Riesz potential defines an inverse for a power of the Laplace operator on Euclidean space. They generalize to several variables the Riemann–Liouville integrals of one variable.

If 0 < α < n, then the Riesz potential Iαf of a locally integrable function f on Rn is the function defined by

where the constant is given by

$c_{\alpha }=\pi ^{n/2}2^{\alpha }{\frac {\Gamma (\alpha /2)}{\Gamma ((n-\alpha )/2)}}.$ This singular integral is well-defined provided f decays sufficiently rapidly at infinity, specifically if f ∈ Lp(Rn) with 1 ≤ p < n/α. If p > 1, then the rate of decay of f and that of Iαf are related in the form of an inequality (the Hardy–Littlewood–Sobolev inequality)

$\|I_{\alpha }f\|_{p^{*}}\leq C_{p}\|f\|_{p},\quad p^{*}={\frac {np}{n-\alpha p}}.$ More generally, the operators Iα are well-defined for complex α such that 0 < Re α < n.

The Riesz potential can be defined more generally in a weak sense as the convolution

$I_{\alpha }f=f*K_{\alpha }\,$ where Kα is the locally integrable function:

$K_{\alpha }(x)={\frac {1}{c_{\alpha }}}{\frac {1}{|x|^{n-\alpha }}}.$ The Riesz potential can therefore be defined whenever f is a compactly supported distribution. In this connection, the Riesz potential of a positive Borel measure μ with compact support is chiefly of interest in potential theory because Iαμ is then a (continuous) subharmonic function off the support of μ, and is lower semicontinuous on all of Rn.

Consideration of the Fourier transform reveals that the Riesz potential is a Fourier multiplier. In fact, one has

${\widehat {K_{\alpha }}}(\xi )=|2\pi \xi |^{-\alpha }$ and so, by the convolution theorem,

${\widehat {I_{\alpha }f}}(\xi )=|2\pi \xi |^{-\alpha }{\hat {f}}(\xi ).$ The Riesz potentials satisfy the following semigroup property on, for instance, rapidly decreasing continuous functions

$I_{\alpha }I_{\beta }=I_{\alpha +\beta }\$ provided

$0<\operatorname {Re\,} \alpha ,\operatorname {Re\,} \beta Furthermore, if 2 < Re α <n, then

$\Delta I_{\alpha +2}=-I_{\alpha }.\$ One also has, for this class of functions,

$\lim _{\alpha \to 0^{+}}(I^{\alpha }f)(x)=f(x).$ 