In this paper, we discuss various properties of the new modulus of smoothness [ omega^varphi_{k,r}(f^{(r)},t)_p := sup_{0 < hleq t}|mathcal W^r_{kh}(cdot) Delta_{hvarphi(cdot)}^k (f^{(r)},cdot)|_{L_p[-1,1]}, ] where $mathcal W_delta(x) = bigl((1-x-de
ltavarphi(x)/2) (1+x-deltavarphi(x)/2)bigr)^{1/2}. $ Related moduli with more general weights are also considered.
We introduce new moduli of smoothness for functions $fin L_p[-1,1]cap C^{r-1}(-1,1)$, $1le pleinfty$, $rge1$, that have an $(r-1)$st locally absolutely continuous derivative in $(-1,1)$, and such that $varphi^rf^{(r)}$ is in $L_p[-1,1]$, where $varph
i(x)=(1-x^2)^{1/2}$. These moduli are equivalent to certain weighted DT moduli, but our definition is more transparent and simpler. In addition, instead of applying these weighted moduli to weighted approximation, which was the purpose of the original DT moduli, we apply these moduli to obtain Jackson-type estimates on the approximation of functions in $L_p[-1,1]$ (no weight), by means of algebraic polynomials. Moreover, we also prove matching inverse theorems thus obtaining constructive characterization of various smoothness classes of functions via the degree of their approximation by algebraic polynomials.
Several results on constrained spline smoothing are obtained. In particular, we establish a general result, showing how one can constructively smooth any monotone or convex piecewise polynomial function (ppf) (or any $q$-monotone ppf, $qgeq 3$, with
one additional degree of smoothness) to be of minimal defect while keeping it close to the original function in the ${mathbb L}_p$-(quasi)norm. It is well known that approximating a function by ppfs of minimal defect (splines) avoids introduction of artifacts which may be unrelated to the original function, thus it is always preferable. On the other hand, it is usually easier to construct constrained ppfs with as little requirements on smoothness as possible. Our results allow to obtain shape-preserving splines of minimal defect with equidistant or Chebyshev knots. The validity of the corresponding Jackson-type estimates for shape-preserving spline approximation is summarized, in particular we show, that the ${mathbb L}_p$-estimates, $pge1$, can be immediately derived from the ${mathbb L}_infty$-estimates.
