Geodesic
Direct and converse theorems of the theory of approximation in the metric of $L_p$ for $0$
Matematičeskie zametki, Tome 18 (1975) no. 5, pp. 641-658.

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A direct theorem of the Jackson type and several converse theorems are established for the approximation of periodic functions of period $2\pi$ by trigonometrical polynomials in the metric of $L_p$, $0$. 
@article{MZM_1975_18_5_a1,
     author = {V. I. Ivanov},
     title = {Direct and converse theorems of the theory of approximation in the metric of $L_p$ for $0<p<1$},
     journal = {Matemati\v{c}eskie zametki},
     pages = {641--658},
     publisher = {mathdoc},
     volume = {18},
     number = {5},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1975_18_5_a1/}
}
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V. I. Ivanov. Direct and converse theorems of the theory of approximation in the metric of $L_p$ for $0