On the best polynomial approximation of $2\pi$-periodic functions in the $L_2$ space

On the classes $L^r_2$, where $r\in {\mathbb{Z}}_+$, exact constants of Jackson type inequalities have been obtained for the characteristics of smoothness ${\Delta}_k (f)$, $k\in \mathbb{N}$, which are defined by the averaged $k$-th order finite differences of functions $f\in L_2$.