“…For a numerical analysis of this approach, and for appropriate eigenvalue solvers for (1.3), see, for example, [1,4,9,14]. Instead of a finite element approach, which always requires a discretization of the computational domain Ω, we will use boundary integral formulations and related boundary element methods [15,16] to solve the eigenvalue problem (1.1). Then, only a discretization of the boundary Γ = ∂Ω is required, and the computational complexity is lower than in a finite element approach when using fast boundary element methods [12].…”