“…These methods appeared long before the ones suggested by Nowak and Hall [44,45], but in the acoustical and electromagnetic wave contexts. As explained in the excellent review articles [4,8,72] (see also [55,53,41,58,19,20,64,42,18,5,63,21]), all these methods are based on employing linear combinations of the extended boundary condition, first-kind integral equation and second-kind integral equations so as to result, after discretization, in a matrix equation whose matrix is not singular at any frequency. In particular, this was the procedure adopted by Brakhage and Werner [14], Schenck [52], Bolomey and Tabbara [11,12], Burton and Miller [15], Mautz and Harrington [38,39], just to name a few.…”