Kushnir, V. P. “Absolute Asymptotic Stability of Solutions of Linear Parabolic Differential Equations With Delay”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, no. 12, Dec. 2007, pp. 1714–1721, https://umj.imath.kiev.ua/index.php/umj/article/view/3426.