Ronto, A. M. 2003. “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 (11), 1541-68. https://umj.imath.kiev.ua/index.php/umj/article/view/4023.