Optimal quadrature problem on classes defined by kernels satisfying certain oscillation properties

Abstract

We consider some classes of 2π-periodic functions defined by a class of operators having certain oscillation properties, which include the classical Sobolev class and a class of analytic functions which can not be represented as a convolution class as its special cases. Let $$\lfloor{x}\rfloor$$ be the largest integer not bigger than x. We prove that on these classes of functions the rectangular formula $$Q^*_N(f) = \frac{2\pi}{N}\sum_{j=0}^{N-1} f\left(\frac{2\pi j}{N}\right)$$ is optimal among all quadrature formulae of the form $$Q_{2N}(f) = \sum_{i=1}^{n}\sum_{j=0}^{\nu_{i}-1}a_{ij}f^{(j)}(t_{i}),$$ where the nodes 0 ≤ t1 < ... < tn < 2π and the coefficients (weights) $$a_{ij}\in \mathbb{R}$$ are arbitrary, i = 1,...,n, j = 0,1,..., νi − 1, and (ν1,...,νn) is a system of positive integers satisfying the condition $$\mathop{\sum}_{i=1}^{n}2\lfloor{(\nu_i+1)/2}\rfloor\leq 2N$$. In particular, the rectangular formula is optimal for these classes of functions among all quadrature formulae of the form $$Q_N(f) = \sum_{i=1}^{N}a_{i}f(t_{i}),$$ with free nodes 0 ≤ t 1 < ... < t N < 2π and arbitrary weights $$a_{i}\in \mathbb{R}, i=1,\ldots,N$$. Moreover, we exactly determine the error estimates of the optimal quadrature formulae on these classes of functions.