2019
Том 71
№ 11

# On Kolmogorov-Type Inequalities Taking into Account the Number of Changes in the Sign of Derivatives

Kofanov V. A.

Abstract

For 2π-periodic functions $x \in L_\infty ^r$ and arbitrary q ∈ [1, ∞] and p ∈ (0, ∞], we obtain the new exact Kolmogorov-type inequality $|| x^(k) ||_q \leqslant (\frac{v(x^(k))}{2})^{1/q} \frac{|| \phi_{r-k} ||_q}{||| \phi_r |||_p^\alpha} ||| x |||_p^\alpha || x^(r) ||_\infty^{1- \alpha}, k, r \in N, k < r,$ which takes into account the number of changes in the sign of the derivatives ν(x (k)) over the period. Here, α = (rk + 1/q)/(r + 1/p), ϕ r is the Euler perfect spline of degree r, $\begin{gathered} \left\| {\left| x \right|} \right\|_p : = {\text{sup}}_{a,b \in {\text{R}}} \{ E_0 (x)_{L_p [a,b]} :x'(t) \ne 0{\text{ }}\forall t \in (a,b)\} , \\ {\text{ }} \\ {\text{ }}E_0 (x)_{L_p [a,b]} : = {\text{ inf}}_{c \in {\text{R}}} \left\| {x - c} \right\|_{L_p [a,b]} , \\ \\ \left\| x \right\|_{L_p [a,b]} : = \left\{ {\int\limits_a^b {\left| {x(t)} \right|^p dt} } \right\}^{1/p} {\text{ for }}0 < p < \infty , \\ \end{gathered}$ and $\left\| x \right\|_{L_p [a,b]} : = {\text{ sup vrai}}_{t \in \left[ {a,b} \right]} \left| {x(t)} \right|$ . The inequality indicated turns into the equality for functions of the form x(t) = aϕ r (nt + b), a, bR, nN. We also obtain an analog of this inequality in the case where k = 0 and q = ∞ and prove new exact Bernstein-type inequalities for trigonometric polynomials and splines.

English version (Springer): Ukrainian Mathematical Journal 55 (2003), no. 4, pp 548-565.

Citation Example: Kofanov V. A. On Kolmogorov-Type Inequalities Taking into Account the Number of Changes in the Sign of Derivatives // Ukr. Mat. Zh. - 2003. - 55, № 4. - pp. 456-469.

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