ShydlichA. L. “Order Equalities for Some Functionals and Their Application to the Estimation of the Best $n$-Term Approximations and Widths”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 61, no. 10, Oct. 2009, pp. 1403-2, https://umj.imath.kiev.ua/index.php/umj/article/view/3110.