@inbook{T2020, author = {T. Stepaniuk,}, editor = {Raigorodskii, A.M. and Rassias, M. Th.}, year = {2020}, pages = {273-287}, title = {Order estimates of best orthogonal trigonometric approximations of classes of infinitely differentiable functions}, publisher = {Springer}, address = {Cham}, journal = {Trigonometric Sums and Their Applications} }