Babenko, V. V., and A. A. Rudenko. 1996. “Optimization of Quadratures on Classes of Functions Given by Differential Operators With Real Spectra”.
Ukrains’kyi Matematychnyi Zhurnal 48 (3): 291-300.
https://umj.imath.kiev.ua/index.php/umj/article/view/5326.