STASYUK, S. A. Best <i>M</i>-term orthogonal trigonometric approximations of the classes <i>B</i><sup> Ω</sup><sub>p,θ</sub&gt; of periodic functions of many variables. Ukrains’kyi Matematychnyi Zhurnal, v. 60, n. 5, p. 647 -, 25 May 2008.