Potential of a mass distribution
An expression of the form
where is a bounded domain in a Euclidean space , , bounded by a closed Lyapunov surface (a curve for , cf. Lyapunov surfaces and curves), is the fundamental solution of the Laplace operator:
where is the area of the unit sphere in , is the distance between the points and , and is the volume element in .
If , then the potential is defined for all and . In the complementary domain , the function then has derivatives of all orders and satisfies the Laplace equation: , that is, is a harmonic function; for this function is regular at infinity, . In the potential belongs to the class and satisfies the Poisson equation: .
These properties can be generalized in various ways. For example, if , then , , in , has generalized second derivatives in , and the Poisson equation is satisfied almost-everywhere in . Properties of potentials of an arbitrary Radon measure concentrated on an -dimensional domain have also been studied:
Here again and in , almost-everywhere in , where is the derivative of with respect to Lebesgue measure in . In definition (*) the fundamental solution of the Laplace operator may be replaced by an arbitrary Levi function (see ) for a general second-order elliptic operator with variable coefficients of class ; then the properties listed above still hold with replaced by (see –).
For the solution of boundary value problems for parabolic partial differential equations the concept of a heat potential of the form
is used, where is a fundamental solution of the heat equation in :
and is the density. The function and its generalizations to the case of an arbitrary second-order parabolic partial differential equation have properties similar to those given above for (see –).
|||N.M. Günter, "Potential theory and its applications to basic problems of mathematical physics" , F. Ungar (1967) (Translated from French)|
|||C. Miranda, "Partial differential equations of elliptic type" , Springer (1970) (Translated from Italian)|
|||A.N. [A.N. Tikhonov] Tichonoff, A.A. Samarskii, "Differentialgleichungen der mathematischen Physik" , Deutsch. Verlag Wissenschaft. (1959) (Translated from Russian)|
|||V.I. Smirnov, "A course of higher mathematics" , 4 , Addison-Wesley (1964) (Translated from Russian)|
|||A. Friedman, "Partial differential equations of parabolic type" , Prentice-Hall (1964)|
|||A.V. Bitsadze, "Boundary value problems for second-order elliptic equations" , North-Holland (1968) (Translated from Russian)|
A Levi function of a linear partial differential equation is also called a fundamental solution of this equation, or a parametrix of this equation. This function is named after E.E. Levi, who anticipated [a1], [a2] what is known today as the parametrix method.
|[a1]||E.E. Levi, "Sulle equazioni lineari alle derivate parziali totalmente ellittiche" Rend. R. Acc. Lincei, Classe Sci. (V) , 16 (1907) pp. 932–938|
|[a2]||E.E. Levi, "Sulle equazioni lineari totalmente ellittiche alle derivate parziali" Rend. Circ. Mat. Palermo , 24 (1907) pp. 275–317|
|[a3]||O.D. Kellogg, "Foundations of potential theory" , F. Ungar (1929) (Re-issue: Springer, 1967)|
Potential of a mass distribution. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Potential_of_a_mass_distribution&oldid=16403