Alles, EJ;    Van Dongen, KWA;      (2011)    Perfectly matched layers for frequency-domain integral equation acoustic scattering problems.                   IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control , 58  (5)   pp. 1077-1086.    10.1109/TUFFC.2011.1908 <https://doi.org/10.1109/TUFFC.2011.1908>.       Green open access