This article is cited in
3 papers
Exact Constants in Generalized Inequalities for Intermediate Derivatives
A. A. Lunev,
L. L. Oridoroga Donetsk National University
Abstract:
Consider the Sobolev space
$W_2^n(\mathbb R_+)$ on the semiaxis with norm of general form defined by a quadratic polynomial in derivatives with nonnegative coefficients. We study the problem of exact constants
$A_{n,k}$ in inequalities of Kolmogorov type for the values of intermediate derivatives
$|f^{(k)}(0)|\le A_{n,k}\|f\|$. In the general case, the expression for the constants
$A_{n,k}$ is obtained as the ratio of two determinants. Using a general formula, we obtain an explicit expression for the constants
$A_{n,k}$ in the case of the following norms:
$$
\|f\|_1^2=\|f\|_{L_2}^2+\|f^{(n)}\|_{L_2}^2\qquad\text{and}\qquad
\|f\|_2^2=\sum_{l=0}^n\|f^{(l)}\|_{L_2}^2.
$$
In the case of the norm
$\|\cdot\|_1$, formulas for the constants
$A_{n,k}$ were obtained earlier by another method due to Kalyabin. The asymptotic behavior of the constants
$A_{n,k}$ is also studied in the case of the norm
$\|\cdot\|_2$. In addition, we prove a symmetry property of the constants
$A_{n,k}$ in the general case.
Keywords:
Sobolev space, Kolmogorov-type inequalities, intermediate derivative, linear functional in Hilbert space, Vandermonde matrix, Cramer's rule.
UDC:
517.518.26 Received: 19.11.2007
Revised: 02.12.2008
DOI:
10.4213/mzm4299