1. E. Schmidt, “Zur Theorie der linearen und nichtlinearen Integralgleichungen I,” Math. Ann. 63(4), 433 (1907).
2. R. DeVore, “Nonlinear Approximation,” Acta Numerica 7, 51 (1998).
3. L. Leindler, “On the Relationships of Seven Numerical Sequences,” Acta math. Acad. sci. hung. 114(3), 227 (2007).
4. S. B. Stechkin, “On Absolute Convergence of Orthogonal Series,” Doklady Akad. Nauk SSSR 102(1), 37 (1955).
5. N. L. Kudryavtsev, “On Connections Between the m-Term and Best Approximations of Some Classes of Sequences in the Spaces lp,” in Functional Spaces. Differential Operators. Problems of Mathematical Education: Abstracts of Reports 5th Int. Conf. Dedicated to 95th Anniversary of Corr. Member of the Russian Academy of Sciences, Acad. European Academy of Sciences L. D. Kudryavtsev (RUDN University, Moscow, 2018), pp. 31–32.