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Heat balls and Fulks measures. (English) Zbl 0592.35057
On every heat sphere $$\Omega$$ (z,c) with center $$z\in {\mathbb{R}}^{n+1}$$ and radius $$c>0$$ the measure $$\mu_{z,c}$$ introduced by W. Fulks [Proc. Am. Math. Soc. 17, 6-11 (1966; Zbl 0152.105)] plays the same role for the solution of the heat equation as the surface measure on the Euclidean sphere does for the solution of the Laplace equation. It is shown that $$\mu_{z,c}$$ is obtained by sweeping the Dirac measure at z on the complement of $$\Omega$$ (z,c) or, in probabilistic terms, that $$\mu_{z,c}$$ is the distribution of the first hit of the heat sphere $$\partial \Omega (z,c)$$ by the space-time Brownian motion starting at z. This characterization of $$\mu_{z,c}$$ implies that the hypertemperatures introduced by N. A. Watson [Proc. Lond. Math. Soc., III. Ser. 26, 385-417 (1973; Zbl 0253.35045); ibid. 33, 251-298 (1976; Zbl 0336.35046)] are the hyperharmonic functions of the harmonic space given by the heat equation.
Reviewer: W.Hansen

##### MSC:
 35K05 Heat equation 35J05 Laplace operator, Helmholtz equation (reduced wave equation), Poisson equation 31D05 Axiomatic potential theory
##### Keywords:
heat equation; Laplace equation; hyperharmonic functions
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