Necessary conditions for the Fourier coefficients of periodic functions to belong to B(p, k, )-classes of Besov type.
We give necessary conditions in terms of the coefficients for the convergence of a double trigonometric series in the -metric, where . The results and their proofs have been motivated by the recent papers of A. S. Belov (2008) and F. Móricz (2010). Our basic tools in the proofs are the Hardy-Littlewood inequality for functions in and the Bernstein-Zygmund inequalities for the derivatives of trigonometric polynomials and their conjugates in the -metric, where .
We consider operators of the form with Ω(y,u) = K(y,u)h(y-u), where K is a Calderón-Zygmund kernel and (see (0.1) and (0.2)). We give necessary and sufficient conditions for such operators to map the Besov space (= B) into itself. In particular, all operators with , a > 0, a ≠ 1, map B into itself.
We present two sufficient conditions for nonconvolution kernels to be of positive type. We apply the results to obtain stability for one-dimensional models of chemically reacting viscoelastic materials.
Let X be a closed subspace of , where μ is an arbitrary measure and 1 < p < ∞. Let U be an invertible operator on X such that . Motivated by applications in ergodic theory, we obtain (optimal) conditions for the convergence of series like , 0 ≤ α < 1, in terms of , generalizing results for unitary (or normal) operators in L²(μ). The proofs make use of the spectral integration initiated by Berkson and Gillespie and, more particularly, of results from a paper by Berkson-Bourgain-Gillespie....