Derkach, V. A., and O. Sukhorukova. “$A$-regular–$A$-Singular Factorizations of Generalized $J$-Inner Matrix Functions”. Methods of Functional Analysis and Topology, vol. 23, no. 3, Sept. 2017, pp. 231-5, https://zen.imath.kiev.ua/index.php/mfat/article/view/667.