“…In the above citations, the given formulas are based on the explicit knowledge of the dual singular solution of the adjoint problem. Furthermore, there exist several algorithms for the numerical computation of the coefficients c k , see for example, Assous et al (1998Assous et al ( , 2000Assous et al ( , 2003a, Blum and Dobrowolski (1982), Bourlard et al (1992), , , Destuynder and Djaoua (1980), Fix et al (1973), Moussaoui (1985), Strang and Fix (1973).…”