J. J. Liu, P. A. Krutitskii & M. Sini. (1970). Numerical Solution of the Scattering Problem for Acoustic Waves by a Two-Sided Crack in 2-Dimensional Space.
            <i>Journal of Computational Mathematics</i>. <i>29</i> (2).
            141-166.
         doi:10.4208/jcm.1006-m3131