Ahangar, Reza. “Stability and Asymptotic Behavior of the Causal Operator Dynamical Systems Using Nonlinear Variation of Parameters”. Journal of Progressive Research in Mathematics 13, no. 1 (January 31, 2018): 2170-2189. Accessed December 25, 2024. http://scitecresearch.com/journals/index.php/jprm/article/view/1364.