A relation between spline approximation and the problem of the approximation of one class by another

An investigation of the approximation in $L_q(-\infty,\,\infty)$ of differentiable functions whose $k$-th derivatives belong to $L_p(-\infty,\,\infty)$, by splines $S_m(x)$ with nonfixed nodes, under the extra assumption that the norms in $L_s(-\infty,\infty)$ of their $l$-th derivatives have a common bound. A relation is established with the problem of approximating functions of one class by functions of another class.