Dang, Dinh Chau, and Tuan Vu. “Asymptotic Equivalence of Triangular Differential Equations in Hilbert Spaces”. Ukrains’kyi Matematychnyi Zhurnal 57, no. 3 (March 25, 2005): 329–337. Accessed May 3, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3601.