  In this paper, we specify a set of optimal subspaces for $L_2$ approximation
of three classes of functions in the Sobolev space $W^{(r)}_2$, defined on a
segment and subject to certain boundary conditions. All of these subspaces are
generated by equidistant shifts of a single function. In particular, we
indicate optimal spline spaces of all degrees $d\geqslant r-1$ with uniform
knots.
