Rogovchenko , S. P. “«Shortening» of Calculating Systems of Differential Equations With the Impulse effeсt”. Ukrains’kyi Matematychnyi Zhurnal, vol. 40, no. 4, June 1988, pp. 481-7, https://umj.imath.kiev.ua/index.php/umj/article/view/9183.