ABSTRACT
Introduction Let R be the set of real numbers, R3 be the three-dimensional real Euclidean space, and x = (x1, x2, x3)T ∈ R3 be a generic vector, where the superscript T denotes transposed. We denote with (·, ·) the Euclidean scalar product in R3 and with ‖ · ‖ the corresponding vector norm. Let R3 be filled with a homogeneous isotropic medium in equilibrium at rest with no source terms present. Let Ω, D, and DG be bounded simply connected open sets contained in R3 with locally Lipschitz boundaries ∂Ω, ∂D, ∂DG and let Ω, D, DG be their closures respectively. We denote with n(x) = (n1(x), n2(x), n3(x))T ∈ R3, x ∈ ∂Ω the outward unit normal vector to ∂Ω in x ∈ ∂Ω. Similarly n(x) = (n1(x), n2(x), n3(x))T ∈ R3, x ∈ ∂D or x ∈ ∂DG denotes the outward unit normal to ∂D in x ∈ ∂D or to ∂DG in x ∈ ∂DG.
