On the Solvability of a Class of Second Kind Integral Equations on Unbounded Domains

AbstractWe consider integral equations of the form ψ(x)=φ(x)+∫Ωk(x,y)z(y)ψ(y)dy(in operator form ψ=φ+Kzψ), where Ω is some subset ofRn(n≥1). The functionsk,z, and φ are assumed known, withz∈L∞(Ω) and φ∈Y, the space of bounded continuous functions on Ω. The function ψ∈Yis to be determined. The class of domains Ω and kernelskconsidered includes the case Ω=Rnandk(x,y)=κ(x−y) with κ∈L1(Rn), in which case, ifzis the characteristic function of some setG, the integral equation is one of Wiener–Hopf type. The main theorems, proved using arguments derived from collectively compact operator theory, are conditions on a setW⊂L∞(Ω) which ensure that ifI−Kzis injective for allz∈WthenI−Kzis also surjective and, moreover, the inverse operators (I−Kz)−1onYare bounded uniformly inz. These general theorems are used to recover classical results on Wiener–Hopf integral operators of21and19, and generalisations of these results, and are applied to analyse the Lippmann–Schwinger integral equation