Exact constants in Jackson–Stechkin inequality in $L^{2}$ with a power-law weight

In this paper, we have solved several extremal problems of the best mean-square approximation of function $f$, on the semiaxis with a power-law weight, which can be used to solve various problems. Sharp Jackson–Stechkin type inequalities are obtained on some classes of functions in which the values of the best approximations are estimated from above through moduli of smoothness of the $k$-th order.