Bushev D. M.
Isometry of the subspaces of solutions of systems of differential equations to the spaces of real functions
Ukr. Mat. Zh. - 2019. - 71, № 8. - pp. 1011-1027
We determine the subspaces of solutions of the systems of Laplace and heat-conduction differential equations isometric to the corresponding spaces of real functions determined on the set of real numbers.
Conditions of Convergence Almost Everywhere for the Convolution of a Function with Delta-Shaped Kernel to this Function
Ukr. Mat. Zh. - 2015. - 67, № 11. - pp. 1461-1476
We establish sufficient conditions for the convergence of the convolution of a function with delta-shaped kernel to this function. These conditions are used for the construction of the subspaces of solutions of differential equations and systems of these equations isometric to the spaces of real functions.
Ukr. Mat. Zh. - 2006. - 58, № 1. - pp. 12–19
In an N-dimensional space, we consider the approximation of classes of translation-invariant periodic functions by a linear operator whose kernel is the product of two kernels one of which is positive. We establish that the least upper bound of this approximation does not exceed the sum of properly chosen least upper bounds in m-and ((N ? m))-dimensional spaces. We also consider the cases where the inequality obtained turns into the equality.
Ukr. Mat. Zh. - 2000. - 52, № 2. - pp. 183-196
We prove that the approximations of classes of periodic functions with small smoothness in the metrics of the spaces C and L by different linear summation methods for Fourier series are asymptotically equal to the least upper bounds of the best approximations of these classes by trigonometric polynomials of degree not higher than (n - 1). We establish that the Fejér method is asymptotically the best among all positive linear approximation methods for these classes.
Ukr. Mat. Zh. - 1998. - 50, № 8. - pp. 1027–1045
We construct spaces of real functions of n + k variables that are isometric to spaces of real functions given on an n-dimensional Euclidean space. We present certain properties and examples of delta-like kernels used for the construction of isometric spaces of functions with different number of variables. We prove certain assertions that enable one to construct delta-like kernels with many variables by using delta-like kernels with smaller number of variables.
Ukr. Mat. Zh. - 1997. - 49, № 6. - pp. 739–753
We consider a family of special linear methods of summation of Fourier series and establish exact equalities for the approximation of classes of convolutions with even and odd kernels by polynomials generated by these methods.
Ukr. Mat. Zh. - 1993. - 45, № 1. - pp. 26–31
Asymptotic equalities are found for the least upper bounds of the best approximations of some convolution classes with even kernel in the metric of a space $L_p$.
Ukr. Mat. Zh. - 1990. - 42, № 3. - pp. 406-412
Asymptotically best approximation of glasses of differentiable functions by linear positive operators
Ukr. Mat. Zh. - 1985. - 37, № 2. - pp. 154 – 162