BrzychczyS., PrykarpatskyA. K., and SamoilenkoV. G. 2001. “Finite-Dimensional Reductions of Conservative Dynamical Systems and Numerical Analysis. I”. Ukrains’kyi Matematychnyi Zhurnal 53 (2), 220-28. https://umj.imath.kiev.ua/index.php/umj/article/view/4237.