  This note is a continuation of our papers [1,2], devoted to $L$-approximation
of characteristic function of $(-h, h)$ by trigonometric polynomials. In the
paper [1] the sharp values of the best approximation for the special values of
$h$ were found. In [2] we gave the complete solution of the problem for
arbitrary values of $h$. In general case [2] the situation is more deep and
results are not so simple as in [1]. For applications to the problem of optimal
constants in the Jackson-type inequalities we need, however, results on
$L$-approximation of $B$-splines and linear combinations of $B$-splines. Here
we present some simple results about $L$-approximation of $B$-splines as well
as give the the proof of its sharpness for the special values of $h$.
