LysenkoZ. M., MatviyukL. V., NechaevL. V., and ShvetsV. T. “On the Fredholm Theory of a Planar Problem With Shift for a Pair of Functions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 63, no. 4, Apr. 2011, pp. 566-71, https://umj.imath.kiev.ua/index.php/umj/article/view/2742.