doiSerbia

Exact constants in direct and inverse approximation theorems for functions of several variables in the spaces Sp

Abdullayev Fahreddin G. (Kyrgyz-Turkish Manas University, Bishkek, Kyrgyz Republic + Mersin University, Mersin, Turkey)
Özkartepe Pelin (Gaziantep University, Gaziantep, Turkey)
Savchuk Viktor V. (Institute of Mathematics of NAS of Ukraine, Kyiv, Ukraine)
Shidlich Andrii L. (Institute of Mathematics of NAS of Ukraine, Kyiv, Ukraine)

In the paper, exact constants in direct and inverse approximation theorems for functions of several variables are found in the spaces Sp. The equivalence between moduli of smoothness and some K-functionals is also shown in the spaces Sp.

Keywords: Direct theorem, inverse theorem, modulus of smoothness, K-functional