Convexity, Moduli of Smoothness and a Jackson-Type Inequality

Abstract

For a Banach space $B$ of functions which satisfies for some $m>0$ $$ \max(\|F+G\|_B,\|F-G\|_B) \ge (\|F\|^s_B + m\|G\|^s_B)^{1/s}, \forall F,G\in B \ (*) $$ a significant improvement for lower estimates of the moduli of smoothness $\omega^r(f,t)_B$ is achieved. As a result of these estimates, sharp Jackson inequalities which are superior to the classical Jackson type inequality are derived. Our investigation covers Banach spaces of functions on $R^d$ or $T^d$ for which translations are isometries or on $S^{d-1}$ for which rotations are isometries. Results for $C_0$ semigroups of contractions are derived. As applications of the technique used in this paper, many new theorems are deduced. An $L_p$ space with $1<p<\infty $ satisfies $(*)$ where $s=\max(p,2),$ and many Orlicz spaces are shown to satisfy $(*)$ with appropriate $s.$