Geodesic
On a~theorem of Jackson
Matematičeskie zametki, Tome 16 (1974) no. 5, pp. 681-690.

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We prove that $$ \inf_{L_n\in Z_n}\sup_\omega\,^*\sup_{f\in H_\omega}\frac{\|f-L_n(f)\|}{\omega(\frac\pi{n+1})}=1\quad(n=0,1,2,\dots), $$ where $\inf\limits_{L_n\in Z_n}$ is taken over all linear polynomial approximation methods of degree not higher than $n$ and $\sup\limits_\omega{}^*$ over all convex moduli of continuity $\omega(\delta)$. 
@article{MZM_1974_16_5_a0,
     author = {A. N. Davidchik and A. A. Ligun},
     title = {On a~theorem of {Jackson}},
     journal = {Matemati\v{c}eskie zametki},
     pages = {681--690},
     publisher = {mathdoc},
     volume = {16},
     number = {5},
     year = {1974},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1974_16_5_a0/}
}
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A. N. Davidchik; A. A. Ligun. On a~theorem of Jackson. Matematičeskie zametki, Tome 16 (1974) no. 5, pp. 681-690. http://geodesic.mathdoc.fr/item/MZM_1974_16_5_a0/