“…The usefulness of these algorithms has been extended in recent years to problems on general bounded regions by the development of capacitance matrix, or imbedding, methods; see Buzbee and Dorr [6], Buzbee, Dorr, George and Golub [7] [42], [43], Proskurowski and Widlund [44], [45], Shieh [46], [47], [48] and Widlund [57]. We refer to Proskurowski and Widlund [44] for a discussion of this development up to the beginning of 1976.…”