RukasovV. I., and StepanetsO. I. 2003. “Best ‘Continuous’ $n$-Term Approximations in the Spaces $S_\phi ^p$”. Ukrains’kyi Matematychnyi Zhurnal 55 (5), 663-70. https://umj.imath.kiev.ua/index.php/umj/article/view/3941.