Komarenko, A. N., and V. A. Trotsenko. “Variational Method for the Solution of Problems of Transmission With the Principal Conjugation Condition”. Ukrains’kyi Matematychnyi Zhurnal, vol. 51, no. 6, June 1999, pp. 762–775, https://umj.imath.kiev.ua/index.php/umj/article/view/4663.