Dang, D. C., and T. Vu. “Asymptotic Equivalence of Triangular Differential Equations in Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, no. 3, Mar. 2005, pp. 329–337, https://umj.imath.kiev.ua/index.php/umj/article/view/3601.