Berngardt, O., and A. Voronov. “A Least Squares Method for Sums of Functions Satisfying the Differential Equations With Polynomial Coefficients”. Numerical Methods and Programming (Vychislitel’nye Metody I Programmirovanie), vol. 4, no. 4, Apr. 2003, pp. 167-71, https://en.num-meth.ru/index.php/journal/article/view/85.