RontoA. M. “Exact Solvability Conditions for the Cauchy Problem for Systems of First-Order Linear Functional-Differential Equations Determined by $(σ_1, σ_2, . , σ_n; τ)$-Positive Operators”. Ukrains’kyi Matematychnyi Zhurnal 55, no. 11 (November 25, 2003): 1541-1568. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4023.