Vol. 5 No. 2 (2013)

A densely defined Hermitian operator $A_0$ with equal defect numbers is considered. Presentable by means of resolvents of a certain maximal dissipative or accumulative extensions of $A_0$, bounded linear operators acting from some defect subspace $\mfn_\gamma$ to arbitrary other $\mfn_\lambda$ are investigated. With their aid are discussed characteristic and Weyl functions. A family of Weyl functions is described, associated with a given self-adjoint extension of $A_0$. The specific property of Weyl function's factors enabled to obtain a modified formulas of von Neumann. In terms of characteristic and Weyl functions of suitably chosen extensions the resolvent of Weyl function is presented explicitly.

Let $R$ be a right Noetherian ring which is also an algebra over $\mathbb{Q}$ ($\mathbb{Q}$ the field of rational numbers). Let $\sigma$ be an automorphism of R and $\delta$ a $\sigma$-derivation of $R$. Let further $\sigma$ be such that $a\sigma(a)\in P(R)$ implies that $a\in P(R)$ for $a\in R$, where $P(R)$ is the prime radical of $R$. In this paper we study minimal prime ideals of Ore extension $R[x;\sigma,\delta]$ and we prove the following in this direction: Let $R$ be a right Noetherian ring which is also an algebra over $\mathbb{Q}$. Let $\sigma$ and $\delta$ be as above. Then $P$ is a minimal prime ideal of $R[x;\sigma,\delta]$ if and only if there exists a minimal prime ideal $U$ of $R$ with $P = U[x;\sigma,\delta]$.

We consider trigonometric interpolations with shifted equidistant nodes and investigate their accuracies depending on the shift parameter. Two different types of interpolations are in the focus of our attention: the Krylov-Lanczos and the rational-trigonometric-polynomial interpolations. The Krylov-Lanczos interpolation performs convergence acceleration of the classical trigonometric interpolation by polynomial corrections. Additional acceleration is achieved by application of rational corrections which contain some extra parameters. In both cases, we derive the exact constants of the asymptotic errors and, based on these estimates, we find the optimal shifts that provide with the best accuracy. Optimizations are performed for the pointwise convergence in the regions away from the endpoints. Asymptotic estimates allow optimal selection of the extra parameters in the rational corrections which provides with additional accuracy. Results of numerical experiments clarify theoretical investigations.

The paper considers convergence acceleration of the quasi-periodic and the quasi-periodic-rational interpolations by application of polynomial corrections. We investigate convergence of the resultant quasi-periodic-polynomial and quasi-periodic-rational-polynomial interpolations and derive exact constants of the main terms of asymptotic errors in the regions away from the endpoints. Results of numerical experiments clarify behavior of the corresponding interpolations for moderate number of interpolation points.