On absolute continuity of the spectrum of periodic Schrödinger operators

In this paper we find a new condition on a real periodic potential for which the self-adjoint Schrödinger operator may be defined by a quadratic form and the spectrum of the operator is purely absolutely continuous. This is based on resolvent estimates and spectral projection estimates in weighted \(L^2\) spaces on the torus, and an oscillatory integral theorem.     

On the variation detracting property of operators of Balázs and Szabados

In this note we spotlight the linear and positive operators of discrete type \({{{(R_n)}_{n\geqq1}}}\) known as Balázs–Szabados operators. We prove that this sequence enjoys the variation detracting property. The convergence in variation of \({{{(R_{n}f)}_{n\geqq1}}}\) to f is also proved. A generalization in Kantorovich sense is constructed and boundedness with respect to BV-norm is revealed.     

Pták function and symmetry

We investigate subadditivity of Pták function as a sufficient and necessary condition for symmetry in the class of involutive spectrally bounded algebras. Among others it is proved an analogue of Raikov's criterion for complete lmc^*-algebras in which the hermitian elements have finite spectral radius. This paper is based on the author's Doctoral Thesis, University of Athens, written under the supervision of Professor M. Fragoulopoulou.     

Pták function and symmetry

We investigate subadditivity of Pták function as a sufficient and necessary condition for symmetry in the class of involutive spectrally bounded algebras. Among others it is proved an analogue of Raikov's criterion for complete lmc^*-algebras in which the hermitian elements have finite spectral radius.     

On the projective Finsler metrizability and the integrability of Rapcsák equation

A. Rapcsák obtained necessary and sufficient conditions for the projective Finsler metrizability in terms of a second order partial differential system. In this paper we investigate the integrability of the Rapcsák system and the extended Rapcsák system, by using the Spencer version of the Cartan-Kähler theorem. We also consider the extended Rapcsák system completed with the curvature condition. We prove that in the non-isotropic case there is a nontrivial Spencer cohomology group in the sequences determining the 2-acyclicity of the symbol of the corresponding differential operator. Therefore the system is not integrable and higher order obstruction exists.     

On the k—smoothness and k—strong smoothness

5l.IntroductionFrompal,er[11,L2],to[3j,tI1efol1owingresultsareobtained:1)LetXbeaBanachspace,thenxes(X)={xeX;llxIl=1}isG-differetiableifandonIyif,foreachyeX,letXI=span(x,y).2)LetXbeaBanachspace,thenxeS(X)isF-differetiableifandonlyifIn[4j,NanchaoxunandWangjianhuaintroducedk-smoothnessandk-strongsmooth-ness.lnthisPaPer,wegive:Theorem1LetXbeaBanachspace,thcnthefollowingstatementsareequivalent.(i)xo6S(X)isk-smooth.(ii)IfX*cyXisk-dimensionalsubspaceandx,eX*,thenwhereU(x,,r)={xeX;l1x-xo…     

The three-cross theorem and the six-cross theorem of Pálfy and Szabó

In 1995, Pálfy and Szabó stated the theorem that a projective space satisfies the six-cross theorem if and only if it is Desarguesian. A mistake in their proof is corrected. Moreover, for projective planes, the three-cross theorem of Pálfy and Szabó is identified as the Reidemeister condition.     

GMRES and the Arioli,Pták,and Strakoš parametrization

In this paper we study the parametrization proposed by Arioli, Pták and Strakoš (BIT Numerical Mathematics, v 38, 1998) for the class of matrices having the same GMRES residual norm convergence curve. We give expressions for the Hessenberg matrix and the orthonormal basis vectors constructed by GMRES as well as for iterates and error vectors. The iterates do not depend on the eigenvalues in the sense that changing the coefficients of the characteristic polynomial in the parametrization does not change the GMRES iterates as well as the residuals. However, the error vectors do depend on these coefficients.     

On a class of Szász-Mirakyan type operators

The actual construction of the Szász-Mirakyan operators and its various modifications require estimations of infinite series which in a certain sense restrict their usefulness from the computational point of view. Thus the question arises whether the Szász-Mirakyan operators and their generalizations cannot be replaced by a finite sum. In connection with this question we propose a new family of linear positive operators.     

On a criterion for continuity and compactness of composition operators acting on α-Bloch spaces

In this Note we give a new criterion for the continuity and compactness of composition operators acting on α-Bloch spaces.     

On generalized Szász-Mirakyan operators of functions of two variables

We introduce certain generalized Szász-Mirakyan operators in exponential weight spaces of functions of two variables and we give approximation theorems for them.     

Pták Homomorphism Theorem Revisited

Rodrigues' extension (1989) of the classical Pták's homomoiphism theorem to a non-necessarily locally convex setting stated that a nearly semi-open mapping between a semi-B-complete space and an arbitrary topoiogical vector space is semi-open. In this paper we study this extension and, as a consequence of the results obtained, provide an improvement of Pták's homomorphism theorem.     

On the inverse resonance problemfor Jacobi operators—uniqueness and stability

We estimate the difference of the coefficients of two Jacobi operators (from a certain class) from knowledge about their eigenvalues and resonances. More specifically, we prove that if eigenvalues and resonances of the two operators in a sufficiently large disk are respectively close, then the coefficients are close too. A uniqueness result for the inverse resonance problem follows as a corollary.     

Large deviations and rates of convergence in the Birkhoff ergodic theorem: From Hölder continuity to continuity

It is established that, for ergodic dynamical systems, upper estimates for the decay of large deviations of ergodic averages for (non-Hölder) continuous almost everywhere averaged functions have the same asymptotics as in the Hölder continuous case. The results are applied to obtaining the corresponding estimates for large deviations and rates of convergence in the Birkhoff ergodic theorem with non-Hölder averaged functions in certain popular chaotic billiards, such as the Bunimovich stadiums and the planar periodic Lorentz gas.     

On the Ramsey-Turán numbers of graphs and hypergraphs

Let t be an integer, f(n) a function, and H a graph. Define the t-Ramsey-Turán number of H, RT _t (n,H, f(n)), to be the maximum number of edges in an n-vertex, H-free graph G with α _t (G) ≤ f(n), where α _t (G) is the maximum number of vertices in a K _t -free induced subgraph of G. Erd?s, Hajnal, Simonovits, Sós and Szemerédi [6] posed several open questions about RT _t (n,K _s , o(n)), among them finding the minimum ? such that RT _t (n,K _t+? , o(n)) = Ω(n ²), where it is easy to see that RT _t (n,K _t+1, o(n)) = o(n ²). In this paper, we answer this question by proving that RT _t (n,K _t+2, o(n)) = Ω(n ²); our constructions also imply several results on the Ramsey-Turán numbers of hypergraphs.