Shydlich, A. L. 2009. “Order Equalities for Some Functionals and Their Application to the Estimation of the Best $n$-Term Approximations and Widths”. Ukrains’kyi Matematychnyi Zhurnal 61 (10), 1403-23. https://umj.imath.kiev.ua/index.php/umj/article/view/3110.