Arushanyan, O.B., and S.F. Zaletkin. 2018. “To the Orthogonal Expansion Theory of the Solution to the Cauchy Problem for Second-Order Ordinary Differential Equations”. Numerical Methods and Programming (Vychislitel’nye Metody I Programmirovanie) 19 (56):178-84. https://doi.org/10.26089/NumMet.v19r216.