Inequalities Refining Relationships Between Mixed Modules of Smoothness In Metrics of $L_p$ and $L_\infty$

Abstract

The problem of estimating the moduli of smoothness of functions from Lq in terms of their moduli of smoothness from Lp is well known. The first stage in the estimation of moduli of smoothness was the study of the properties of functions from the Lipschitz classes and obtaining the corresponding embeddings in the works of Titchmarsh, Hardy, Littlewood, Nikol'skii.

The classical Hardy-Littlewood embedding for Lipschitz spaces can be obtained as a consequence of the Ulyanov's inequality for the moduli of continuity of a function of one variable. In the works of Ulyanov, the modulus of smoothness of natural order was considered. The introduction of fractional moduli of smoothness made it possible in the works of Potapov, Simonov, Tikhonov to strengthen the Ulyanov's inequality. Later, the same authors were able to generalize Ulyanov's inequality to functions of two variables, obtaining estimates for mixed moduli of smoothness. The sharpness of these inequalities was proved in the case when $1<p<q<\infty$ or $1=p<q=\infty$.

In this article, we study mixed moduli of smoothness of fractional orders of a function of two variables. Inequalities are obtained that refine the previously known estimates of the Ulyanov type inequalities between mixed moduli of smoothness in the metrics Lp and Lq for values $1<p<q=\infty$. The accuracy of the obtained estimates is investigated. The relationship between these and previously known estimates has been studied.