• In the exterior unbounded homogeneous medium Ωc := Rm Ω1, for a given functionf inp∈ H1/2(Γ) we seek a scattered field ω˜ satisfying. ∆ω˜ + k2ω˜ = 0, in Ωc, . γΓω˜ 1= f inp, (2.4) . ∂rω˜ − ikω˜ = o(|r|(m−1)/2).Unlike problem (2.2), (2.4) is always uniquely solvable [36]. We define the associated solution operator KDΓ asKDΓf inp := ω|D, (2.5) with special attention to KΩc Γf inp Γ ˜and KΣΓf inp, namely the scattered field ω satisfy- 1 Γ Γing (2.4) and its trace γΣωThe decomposition framework that we propose for the continuous problem is the following:1. Solve the interface boundary integral system to find (fΣ, fΓ), using data (γΣuinc, γΓuinc) : . fΣ − KΣΓfΓ = γΣuinc (2.6a) −KΓΣfΣ + fΓ = −γΓuinc2. Construct the total field for the model problem (2.1) using the solution (fΣ, fΓ) of (2.6a), by solving the auxiliary models (2.2) and (2.4): u := KΩ2ΣfΣ, in Ω2,KΩc ΓfΓ + uinc, in Ωc. (2.6b) 1 1We claim that, provided (2.6a) is solvable, the decomposed framework-based field u defined in (2.6b) is the solution of (2.1). Notice that we are implicitly assuming in (2.6b) thatKΩ ΣfΣ = uinc|Ω + KΩ ΓfΓ, (2.7)where we recall the notation Ω12 = Ωc Ω2. Indeed, in view of (2.6a), both functions in (2.7) agree on Σ Γ (the boundary of Ω12). Assuming, as we will do from now on, that the only solution to the homogeneous system ∆v + k2v = 0, in Ω12,. γΓv = 0, γΣv = 0 (2.8) is the trivial one and noticing that n Ω12 1 which implies that KΩ12ΣfΣ and KΩ12ΓfΓ are solutions of the Helmholtz equation in Ω12, we can conclude that (2.7) holds. Since u definedin (2.6b) belongs to H1 (Rm), it is simple to check that this function is the solution of (2.1).We remark that the hypothesis we have taken on the artificial boundaries/domains, i.e. the well-posedness of problems (2.2) and (2.8), are not very restrictive in practice: Σ or Γ can be modified if needed. Alternatively, one can consider different boundary conditions on Γ and Σ (such as Robin conditions), redefining KDΣ and KDΓ accordingly, which will lead to a variant of the framework that we analyze in this article. In a future work we shall explore other boundary conditions on the interfaces and analysis of the resulting variant models.