Babenko, V. V., and A. A. Rudenko. “Optimization of Quadratures on Classes of Functions Given by Differential Operators With Real Spectra”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 48, no. 3, Mar. 1996, pp. 291-00,
https://umj.imath.kiev.ua/index.php/umj/article/view/5326.