Abstract. We consider a d-dimensional random field u = (u(x), x ∈ D) that solves a system of elliptic stochastic equations on a bounded domain D ⊂ R k , with additive white noise and spatial dimension k = 1, 2, 3. Properties of u and its probability law are proved. For Gaussian solutions, using results from [9], we establish upper and lower bounds on hitting probabilities in terms of the Hausdorff measure and Bessel-Riesz capacity, respectively. This relies on precise estimates on the canonical distance of the process or, equivalently, on L 2 estimates of increments of the Green function of the Laplace equation.Keywords: Systems of stochastic Poisson equations; hitting probabilities, capacity; Hausdorff measure. AMS Subject Classification. Primary: 60H15, 60G15, 60J45; Secondary: 60G60, 60H07.
IntroductionLet D be a bounded domain of R k , k = 1, 2, 3, for which the divergence theorem holds. Consider the following system of elliptic stochastic partial differential equations,,j≤d is a non-singular matrix with real-valued entries.The main motivation of this paper has been to find upper and lower bounds for the hitting probabilities P{u(I) ∩ A = ∅}, I ⊂ D, A ⊂ R d , in terms of the Hausdorff measure and the capacity of the set A, respectively. This is a fundamental problem in probabilistic potential theory that, in the context of stochastic partial differential equations, has been extensively studied for the stochastic heat and wave equations. We refer the reader to , for numerical approximations, among others. We observe that in (1), the stochastic forcing is an additive noise. Therefore, in the integral formulation of the system given in (6), the stochastic integral term contains a deterministic integrand and defines a Gaussian process. Since there is no time parameter in (1), considering a multiplicative noise would require a choice of anticipating stochastic integral in (6). For example, one could take the Skorohod integral. This would make the objective of this article difficult and rather speculative.The content of the paper is as follows. In Section 2, we prove the existence and uniqueness of a solution to (1), when the function f satisfies a monotonicity condition (see Theorem 2.2). This is a d-dimensional stochastic process indexed bȳ D, the closure of the domain D, with continuous sample paths and vanishing at the boundary of D, a.s. The proof applies standard methods of the theory of nonlinear monotone operators. In order to make the article self-contained, we include the details of the proof. In Section 3, we prove some properties of the solution to (1). With the a priori bound proved in Proposition 3.1, we prove that the solution lies in L p (Ω; R d ), uniformly in x ∈ D. Moreover, by using estimates of increments of the L 2 -norms of the Green function, we prove that the sample paths of the solution are Hölder continuous. Section 3 is devoted to study some aspects of the law of the solution. The integral formulation (6) suggests that the law of u is obtained from a Gaussian process by a non adapted ...