A note on the existence of extension operators for Sobolev spaces on periodic domains

In this note, we prove the existence of a family of extension operators for Sobolev spaces defined on ε-periodic domains. The norms of the operators are shown to be independent of ε. This extension theorem is relevant in the theory of homogenization for PDE's under flux boundary conditions.     

Triplet extensions I: Semibounded operators in the scale of Hilbert spaces

The extension theory for semibounded symmetric operators is generalized by including operators acting in a triplet of Hilbert spaces. We concentrate our attention on the case where the minimal operator is essentially self-adjoint in the basic Hilbert space and construct a family of its self-adjoint extensions inside the triplet. All such extensions can be described by certain boundary conditions, and a natural counterpart of Krein’s resolvent formula is obtained.     

Analyticity of the Schr?dinger propagator on the Heisenberg group

We discuss the analytic extension property of the Schr?dinger propagator for the Heisenberg sublaplacian and some related operators. The result for the sublaplacian is proved by interpreting the sublaplacian as a direct integral of an one parameter family of dilated special Hermite operators.     

Holomorphic extensions of Laplacians and their determinants

The Teichmüller space Teich(S) of a surface S in genus g>1 is a totally real submanifold of the quasifuchsian space QF(S). We show that the determinant of the Laplacian det^′(Δ) on Teich(S) has a unique holomorphic extension to QF(S). To realize this holomorphic extension as the determinant of differential operators on S, we introduce a holomorphic family {Δμ,ν} of elliptic second order differential operators on S whose parameter space is the space of pairs of Beltrami differentials on S and which naturally extends the Laplace operators of hyperbolic metrics on S. We study the determinant of this family {Δμ,ν} and show how this family realizes the holomorphic extension of det^′(Δ) as its determinant.     

GBS Operators of Lupaş–Durrmeyer Type Based on Polya Distribution

In this paper, we study an extension of the bivariate Lupa?–Durrmeyer operators based on Polya distribution. For these operators we get a Voronovskaja type theorem and the order of approximation using Peetre’s K-functional. Then, we construct the Generalized Boolean Sum operators of Lupa?–Durrmeyer type and estimate the degree of approximation in terms of the mixed modulus of smoothness.     

On delay and asymmetry points of one-dimensional diffusion processes

A homogeneous second order linear differential equation is considered. On an open interval where the equation has sense, it generates a family of operators of the Dirichlet problem on the set of all subintervals; this family is a generalized semi-group. Let the equation be defined on two disjoint intervals with a common boundary point z. It is shown that an extension of the corresponding two semi-groups of operators to the semi-group of operators corresponding to the union of the intervals and the point z is not unique and depends on two abritrary constants. To give an interpretation of these arbitrary constants, we use a one-dimensional locally Markov diffusion process with special properties of passage of the point z. One of these arbitrary constants determines the delay of the process at the point z, and the second one induces an asymmetry of the process with respect to z. The two extremal values of the latter constant, 0 and ∞, determine the reflection of the process from the point z when the process approaches the point from the left and right, respectively. Bibliography: 4 titles.     

The Uniform Norming of Retractions on Short Intervals for Certain Function Spaces

For Lizorkin–Triebel spaces the family of extension operators is constructed which yield a minimal (in order) value of the norm among all possible extensions of a given function defined initially on the interval of an arbitrary small length.The techniques used restrict us to the one-dimensional case and spaces defined via differences of first order.     

The Friedrichs Extension of a Pair of Operators

Abstract

An extension of a pair of linear unbounded operators which map from a Banach space X to a Hilbert space Y is constructed and studied. The purpose of the extension is to obtain a pair of jointly closed operators which will be the generating pair of a B-evolution similar to the classical Friedrichs extension of a single operator which generates a holomorphic semigroup. The construction is based on spectral methods.     

Approximation et surjectivité d'opérateurs de type monotone

Summary Brezis-Crandall-Pazy have proved how to approach a maximal monotone operator in a reflexive Banach space by a family of monotone hemicontinuous operators defined on the whole space. An extension to the nonreflexive case, with applications is developped.

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Entrata in Redazione il 14 ottobre 1976.     

紧区间上高斯核近似逼近误差估计的改进

本文研究了由高斯核构成的拟插值算子在闭区间上的近似逼近问题.利用函数延拓和近似单位分划的方法,构造了拟插值算子,并得到了一致范数下的逼近阶估计.     

Exceptional Laurent biorthogonal polynomials through spectral transformations of generalized eigenvalue problems

A formulation is given for the spectral transformation of the generalized eigenvalue problem through the decomposition of the second-order differential operators. This allows us to construct some Laurent biorthogonal polynomial systems with gaps in the degree of the polynomial sequence. These correspond to an exceptional-type extension of the orthogonal polynomials, as an extension of the Laurent biorthogonal polynomials. Specifically, we construct the exceptional extension of the Hendriksen–van Rossum polynomials, which are biorthogonal analogs of the classical orthogonal polynomials. Similar to the cases of exceptional extensions of classical orthogonal polynomials, both state-deletion and state-addition occur.     

Extensions of L d -Loewner chains to higher dimensions

Known results concerning the extension of normalized Loewner chains defined on the unit disk or the euclidean unit ball to higher dimensions, using either a modified Roper-Suffridge extension operator or the Pfaltzgraff-Suffridge extension operator, are shown to hold true in the more general case of L ^d -Loewner chains. Associated to each L ^d -Loewner chain on the unit ball, d ∈ [1,∞], is an evolution family and, as we show holds for the case d < ∞, a Herglotz vector field. We consider these with regard to the extended Loewner chains. To accommodate non-normalized mappings and chains, branches of extension operators are developed. As a corollary to our results, we find that these extension operators also preserve the property of starlikeness of the range of a biholomorphic mapping with respect to the point 0 lying in the closure of the range. We consider how these results can be generalized to the setting of complex Hilbert spaces and conclude with several examples.     

Extension operators via semigroups

The Roper-Suffridge extension operator and its modifications are powerful tools to construct biholomorphic mappings with special geometric properties. The first purpose of this paper is to analyze common properties of different extension operators and to define an extension operator for biholomorphic mappings on the open unit ball of an arbitrary complex Banach space. The second purpose is to study extension operators for starlike, spirallike and convex in one direction mappings. In particular, we show that the extension of each spirallike mapping is A-spirallike for a variety of linear operators A. Our approach is based on a connection of special classes of biholomorphic mappings defined on the open unit ball of a complex Banach space with semigroups acting on this ball.     

Partially Commutative Moment Problems

Partially commutative tensor algebras occur naturally in the algebraic formulation of Wightman field theory. A state on an algebra of this type leads via GNS-construction to a partially commutative family of hermitean operators on Hilbert space. We discuss the question when these operators can be extended to self adjoint operators preserving the commutation properties and state a necessary and sufficient condition for the existence of such an extension in terms of a positivity property of the state.     

Invariant Subspaces for Semigroups of Algebraic Operators

T. Laffey showed (Linear and Multilinear Algebra6(1978), 269–305) that a semigroup of matrices is triangularizable if the ranks of all the commutators of elements of the semigroup are at most 1. Our main theorem is an extension of Hthis result to semigroups of algebraic operators on a Banach space. We also obtain a related theorem for a pair {A, B} of arbitrary bounded operators satisfying rank (AB−BA)=1 and several related conditions. In addition, it is shown that a semigroup of algebraically unipotent operators of bounded degree is triangularizable.     

L-Fuzzy闭包算子的扩张原理

Ｆｕｚｚｙ闭包算子的扩张原理揭示了Ｆｕｚｚｙ闭包算子与经典闭包算子之间的密切关系,是利用传统学科已有结论研究Ｆｕｚｚｙ数学相关理论的有效工具。本文讨论了当Ｌ为有限分配格时,Ｌ－Ｆｕｚｚｙ闭包算子与闭包系统的扩张问题,并给出一种具体的由经典闭包算子生成Ｌ－Ｆｕｚｚｙ闭包算子的方法及其部分性质。     

Essentially subnormal operators

An operator is essentially subnormal if its image in the Calkin algebra is subnormal. We shall characterize the essentially subnormal operators as those operators with an essentially normal extension. In fact, it is shown that an essentially subnormal operator has an extension of the form ``normal plus compact'.

The essential normal spectrum is defined and is used to characterize the essential isometries. It is shown that every essentially subnormal operator may be decomposed as the direct sum of a subnormal operator and some irreducible essentially subnormal operators. An essential version of Putnam's Inequality is proven for these operators. Also, it is shown that essential normality is a similarity invariant within the class of essentially subnormal operators. The class of essentially hyponormal operators is also briefly discussed and several examples of essentially subnormal operators are given.

     

A new iterative algorithm for common solutions of a finite family of accretive operators

The purpose in this paper is to prove a theorem of strong convergence to a common solution for a finite family of accretive operators in a strictly convex Banach space by means of a new iterative algorithm, which is a generalization and extension of the results of Kim and Xu [T.H. Kim, H.K. Xu, Strong convergence of modified Mann iterations, Nonlinear Anal. 61 (2005) 51–60], and Zegeye and Shahzad [H. Zegeye, N. Shahzad, Strong convergence theorems for a common zero of a finite family of m-accretive mappings, Nonlinear Anal. 66 (2007) 1161–1169]. Further using the result, the theorem of strong convergence to a common fixed point is discussed for a finite family of pseudocontractive mappings under certain conditions.     

Generalized Virasoro Operators

Abstract

We define generalized Virasoro operators acting on a Fock space V(Γ). These generalize the standard construction of Virasoro operators. By using the Jacobi Identity we compute the commutators of these operators. These operators result in an abelian extension of the toroidal Lie algebra. We explicitly describe the abelian extension.     

Enumerations, countable structures and Turing degrees

It is proven that there is a family of sets of natural numbers which has enumerations in every Turing degree except for the recursive degree. This implies that there is a countable structure which has representations in all but the recursive degree. Moreover, it is shown that there is such a structure which has a recursively represented elementary extension.