StasyukS. A. 2016. “Best $m$-Term Trigonometric Approximation for Periodic Functions With Low mixed smoothness from the Nikol’skii – Besov-Type Classes”. Ukrains’kyi Matematychnyi Zhurnal 68 (7), 983-1003. https://umj.imath.kiev.ua/index.php/umj/article/view/1896.