On a generalization of the method of monotone operators

For a family of operator equations of the first kind with nonlinear nonmonotone hemicontinuous operators in a reflexive Banach space, we prove a theorem on the solvability and a uniform estimate of the solution in the norm of the space. Our approach is related to the method of semimonotone operators but has some essential differences from the latter. In a specific example, we show that our theorem can be used to prove the total (with respect to the set of admissible controls) preservation of solvability for distributed control systems.     

Existence results for some quasilinear parabolic problems

We first give an existence theorem, for some equations associated with Leray-Lions operators, assuming the existence of a subsolution smaller than a supersolution. Then we prove, with an additional hypothesis on the operator, that in the previous theorem, we can replace the subsolution by two subsolutions and the supersolution by two supersolutions. Finally, we deduce the existence of a smallest and a greatest solution.     

Optimal Secant-Type Methods for Operator Equations

We prove an existence and uniqueness theorem for operator equations in Banach spaces with (generally non-differentiable) operators whose divided differences are Lipschitz continuous on operator's domain. The theorem makes possible to apply the concept of entropy optimality of iterative methods introduced in our earlier work to the class of secant-type methods. Using this concept, we show that it is feasible to get a method that needs the same information (one value of the operator) per iteration but exhibits a faster convergence than the secant and secant-update methods.     

Semigroup-theoretic proofs of the central limit theorem and other theorems of analysis

The theory of one parameter semigroups of bounded linear operators on Banach spaces has deep and far reaching applications to partial differential equations and Markov processes. Here we present some known elementary applications of operator semigroups to approximation theory, a new proof of the central limit theorem, and we give E. Nelson's rigorous interpretation of Feynman integrals. Our main tools are (i) a special case of the Trotter-Neveu-Kato approximation theorem, of which we give a new elementary proof, and (ii) P. Chernoff's product formula.     

On Some Operators Associated to Superoscillations

In this paper we study the convergence of some sequences of operators associated to the Aharonov and Berry’s superoscillating functions. The main tool to define the sequences of operators is the spectral theorem. In particular we discuss the case of sequences of unbounded self-adjoint operators on a Hilbert space. We apply our results to the case where T is the self-adjoint extension of the momentum operator with unbounded spectrum.     

Beurling-Pollard type theorems

We establish a version of the Beurling–Pollard theoremfor operator synthesis and apply it to derive some results onlinear operator equations and to prove a Beurling–Pollardtype theorem for Varopoulos tensor algebras. In addition, weestablish a Beurling–Pollard theorem for weighted Fourieralgebras and use it to obtain ascent estimates for operatorsthat are functions of generalized scalar operators.     

Estimates for restrictions of monotone operators on the cone of decreasing functions in Orlicz space

We introduce a number of notions related to the Lyapunov transformation of linear differential operators with unbounded operator coefficients generated by a family of evolution operators. We prove statements about similar operators related to the Lyapunov transformation and describe their spectral properties. One of the main results of the paper is a similarity theorem for a perturbed differential operator with constant operator coefficient, an operator which is the generator of a bounded group of operators. For the perturbation, we consider the operator ofmultiplication by a summable operator function. The almost periodicity (at infinity) of the solutions of the corresponding homogeneous differential equation is established.     

On Positivity Conditions for the Cauchy Function of Functional-Differential Equations

We study how the statements on estimates of solutions to linear functional-differential equations, analogous to the Chaplygin differential inequality theorem, are connected with the positivity of the Cauchy function and the fundamental solution. We prove a comparison theorem for the Cauchy functions and the fundamental solutions to two functional-differential equations. In the theorem, it is assumed that the difference of the operators corresponding to the equations (and acting from the space of absolutely continuous functions to the space of summable ones) is a monotone totally continuous Volterra operator. We also obtain the positivity conditions for the Cauchy function and the fundamental solution to some equations with delay as long as those of neutral type.     

Fully nonlinear elliptic equations and the dini condition

Abstract

We investigate transmission problems with strongly Lipschitz interfaces for the Dirac equation by establishing spectral estimates on an associated boundary singular integral operator, the rotation operator. Using Rellich estimates we obtain angular spectral estimates on both the essential and full spectrum for general bi-oblique transmission problems. Specializing to the normal transmission problem, we investigate transmission problems for Maxwell's equations using a nilpotent exterior/interior derivativeoperator. The fundamental commutation properties for this operator with the two basic reflection operators are proved. We show how the L ₂spectral estimates are inherited for the domain of the exterior/interior derivative operator and prove some complementary eigenvalue estimates. Finally we use a general algebraic theorem to prove a regularity property needed for Maxwell's equations.     

A fixed-point theorem for S-type operators on Banach spaces and its applications to boundary-value problems

In this article we present a fixed-point theorem of operator equations defined in cones of Banach spaces, where the spectral radius of the operator is involved. The operator is an S-type operator, a meaning introduced here. As an application of our main theorem, we get the existence of positive solutions to two boundary-value problems, extending several results from the literature.     

Unitarizable representations and fixed points of groups of biholomorphic transformations of operator balls

We show that the open unit ball of the space of operators from a finite-dimensional Hilbert space into a separable Hilbert space (we call it “operator ball”) has a restricted form of normal structure if we endow it with a hyperbolic metric (which is an analogue of the standard hyperbolic metric on the unit disc in the complex plane). We use this result to get a fixed point theorem for groups of biholomorphic automorphisms of the operator ball. The fixed point theorem is used to show that a bounded representation in a separable Hilbert space which has an invariant indefinite quadratic form with finitely many negative squares is unitarizable (equivalent to a unitary representation). We apply this result to find dual pairs of invariant subspaces in Pontryagin spaces. In Appendix A we present results of Itai Shafrir about hyperbolic metrics on the operator ball.     

Strong convergence studied by a hybrid type method for monotone operators in a Banach space

In this paper, we study a strong convergence for monotone operators. We first introduce the hybrid type algorithm for monotone operators. Next, we obtain a strong convergence theorem (Theorem 3.3) for finding a zero point of an inverse-strongly monotone operator in a Banach space. Finally, we apply our convergence theorem to the problem of finding a minimizer of a convex function.     

Traces of operators with a relatively compact perturbation

This is a continuation of the authors’ series of papers on the theory of regularized traces of abstract discrete operators. We prove a theorem in which the perturbing operator B is subordinate to the operator A ₀ in the sense that BA ₀ ^?δ is a compact operator belonging to some Schatten-von Neumann class of finite order. Apart from covering new classes of operators, and in contrast to our preceding papers, we give a unified statement of the theorem regardless of whether the resolvent of the unperturbed operator belongs to the trace class. Two examples are given in which the result is applied to ordinary differential operators as well as to partial differential operators.     

Linear equations of the Sobolev type with integral delay operator

We establish sufficient conditions of the local and global solvability of initial value problems for a class of linear operator-differential equations of the first order in a Banach space. Equations are assumed to have a degenerate operator at the derivative and an integral delay operator. We apply methods of the theory of degenerate semigroups of operators and the contraction mapping theorem. As examples illustrating the general results we consider the evolution equation for a free surface of a filtered liquid with a delay and a linearized quasistationary system of equations for a phase field with a delay.     

Concerning the convergence of inexact Newton methods

In this study, we use inexact Newton methods to find solutions of nonlinear operator equations on Banach spaces with a convergence structure. Our technique involves the introduction of a generalized norm as an operator from a linear space into a partially ordered Banach space. In this way the metric properties of the examined problem can be analyzed more precisely. Moreover, this approach allows us to derive from the same theorem, on the one hand, semi-local results of Kantorovich type, and on the other hand, global results based on monotonicity considerations. By imposing very general Lipschitz-like conditions on the operators involved, on the one hand, we cover a wider range of problems, and on the other hand, by choosing our operators appropriately, we can find sharper error bounds on the distances involved than before. Furthermore, we show that special cases of our results reduce to the corresponding ones already in the literature. Finally, our results are used to solve integral equations that cannot be solved with existing methods.     

Generalized conditions for the convergence of inexact Newton-like methods on banach spaces with a convergence structure and applications

In this study, we use inexact Newton-like methods to find solutions of nonlinear operator equations on Banach spaces with a convergence structure. Our technique involves the introduction of a generalized norm as an operator from a linear space into a partially ordered Banach space. In this way the metric properties of the examined problem can be analyzed more precisely. Moreover, this approach allows us to derive from the same theorem, on the one hand, semi-local results of Kantorovich-type, and on the other hand, global results based on monotonicity considerations. By imposing very general Lipschitz-like conditions on the operators involved, on the one hand, we cover a wider range of problems, and on the other hand, by choosing our operators appropriately we can find sharper error bounds on the distances involved than before. Furthermore, we show that special cases of our results reduce to the corresponding ones already in the literature. Finally, our results are used to solve integral equations that cannot be solved with existing methods.     

Solving evolutionary-type differential equations and physical problems using the operator method

We present a general operator method based on the advanced technique of the inverse derivative operator for solving a wide range of problems described by some classes of differential equations. We construct and use inverse differential operators to solve several differential equations. We obtain operator identities involving an inverse derivative operator, integral transformations, and generalized forms of orthogonal polynomials and special functions. We present examples of using the operator method to construct solutions of equations containing linear and quadratic forms of a pair of operators satisfying Heisenberg-type relations and solutions of various modifications of partial differential equations of the Fourier heat conduction type, Fokker–Planck type, Black–Scholes type, etc. We demonstrate using the operator technique to solve several physical problems related to the charge motion in quantum mechanics, heat propagation, and the dynamics of the beams in accelerators.     

Holomorphe Störungstheorie in lokalkonvexen Räumen

In this paper we study bounded holomorphic perturbations of a semi-Fredholm operator between sequentially complete locally convex spaces; however, some results are new in the case of Banach spaces, too. We define a concept of holomorphy for bounded operator functions and show that a meromorphy theorem is true for such perturbations of the identity. Then we deal with the problem when a weakly holomorphic bounded operator function is holomorphic in the defined sense. In the case of one complex variable we then prove an existence and extension theorem for solutions of equations T(z)x=y(z) which answers a question of B. Gramsch [7]. Finally we apply our results to partial differential operators.     

Study on Sobolev type Hilfer fractional integro-differential equations with delay

In this paper, we deal with a class of nonlinear Sobolev type fractional integro-differential equations with delay using Hilfer fractional derivative, which generalized the famous Riemann–Liouville fractional derivative. The definition of mild solutions for studied problem was given based on an operator family generated by the operator pair (A, B) and probability density function. Combining with the techniques of fractional calculus, measure of noncompactness and fixed point theorem, we obtain new existence result of mild solutions with two new characteristic solution operators and the assumptions that the nonlinear term satisfies some growth condition and noncompactness measure condition. The results obtained improve and extend some related conclusions on this topic. At last, an example is given to illustrate our main results.