On the approximation by algebraic polynomials of functions of class $L^p$, $0<p<1$

In the article, Whitney's inequality
$$ E_{k-1}(f;[a,b])\leqslant M_k\omega_k\biggl(\frac{b-a}k,f\biggr) $$
for bounded functions $f$ on $[a,b]$ is extended to the case $f\in L^p(a,b)$ for $0<p<1$.
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