BrzychczyS., PrykarpatskyA. K., and SamoilenkoV. G. “Finite-Dimensional Reductions of Conservative Dynamical Systems and Numerical Analysis. I”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 53, no. 2, Feb. 2001, pp. 220-8, https://umj.imath.kiev.ua/index.php/umj/article/view/4237.