Slyusarchuk, V. Y. “Necessary and Sufficient Conditions for the Absolute Instability of Solutions of linear Differential-Difference Equations With Self-Adjoint Operator Coefficients”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 70, no. 5, May 2018, pp. 715-24, https://umj.imath.kiev.ua/index.php/umj/article/view/1590.