[1]
A. M. Ronto, “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”, Ukr. Mat. Zhurn., vol. 55, no. 11, pp. 1541-1568, Nov. 2003.