Recent Submissions
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Dmitrović, Dušica (Beograd , 2026)[more][less]
Abstract: The study of integral means of the composition of functions defined on the unit disk D in the complex plane dates back to the 1920s, with one of the earliest results in this area being Littlewood’s subordination principle. When investigating the norm of composition operators on certain spaces of holomorphic functions, a natural need arises to study the relationship between the integral means of the composition f ◦ φ and those of the function f itself. Littlewood’s principle is one of the main tools used to establish this connection. However, it is not the only one. In this dissertation, additional methods for studying the relationship between these integral means are presented. By applying these methods, two-sided estimates for the norm of the composition operator Cφ on spaces of mixed norm Hp,q,α are obtained in the form K1 ≤ ∥ Cφ ∥Hp,q,α→Hp,q,α ≤ K2, where the constants K1 and K2 depend on the parameters p, q, α and |φ(0)|. Furthermore, the monotonicity of the integral mean of a holomorphic function f on the unit disk D, denoted by Mp,q,α[f ](ρ, R, s) , is investigated, where 0 < p, q, α < ∞, 0 ≤ ρ < R ≤ 1 and 0 ≤ s ≤ 1. One consequence of this result is the monotonicity of the norm ∥f ∥p,q,α in mixed norm spaces with respect to the parameters p, q, α. One of the operators that can be represented as an integral of weighted composition operators Tt is the Hilbert matrix operator H acting on the weighted Bergman spaces Ap γ . Moreover, it is known that the operator H is bounded if and only if 1 < γ + 2 < p, and in this case, the following lower bound for the norm of the operator holds: ∥H∥Ap γ →Ap γ ≥ π/ sin (γ+2)π p . When γ > 0 and p ≥ 2(γ + 2), it is known that the norm is equal to this constant. In studying the norm of the operator H, after applying Minkowski’s theorem, the application of Minkowski’s inequality reduces the problem to estimating the norm of the operator Tt. As a result of this analysis, in the case where γ < 0 a new upper bound for the norm of the operator H is obtained, while in the case where γ > 0, the interval on which the norm equals the constant π/ sin (γ+2)π p is extended. Finally, the dissertation presents a refinement of Littlewood’s subordination principle under an additional injectivity assumption, together with applications of the new inequality to the Rogosinski theorem and to norm estimates for compositions of functions on weighted Bergman spaces. URI: http://hdl.handle.net/123456789/5783 Files in this item: 1
Dusica_Dmitrovic_doktorska_disertacija.pdf ( 1.895Mb ) -
Aleksić, Danijel (Beograd , 2026)[more][less]
Abstract: This dissertation addresses the problem of model specification testing in situa- tions where data are incomplete, utilizing the existing theory of non-degenerate and weakly degenerate U- and V-statistics. The first two chapters lay the theoretical groundwork by pre- senting essential concepts related to U- and V-statistics and the general mathematical frame- work of missing data analysis, which serve as the foundation for the new results developed in subsequent chapters. In Chapter 3, a novel test for assessing the missing completely at random (MCAR) assump- tion is introduced. This test demonstrates improved control of the type I error rate and supe- rior power performance compared to the main competitor across the majority of the simulated scenarios examined. Chapter 4 explores the application of Kendall’s test for independence in the presence of MCAR data. It provides both theoretical insights and simulation-based comparisons of the complete-case analysis and median imputation, pointing out their individual advantages and drawbacks. Chapter 5 focuses on testing for multivariate normality when data are incomplete. It rig- orously establishes the validity of the complete-case approach under MCAR and proposes a bootstrap method to approximate p -values when imputation is employed. Additionally, vari- ous imputation techniques are evaluated with respect to their impact on the type I error and the power of the test. Finally, Chapter 6 adapts the energy-based two-sample test to handle missing data by intro- ducing a weighted framework that makes full use of all available observations. Alongside some theoretical developments, the chapter presents two distinct bootstrap algorithms for p -value estimation under this approach. Additionally, the performance of several imputation methods is examined in this context, and appropriate bootstrap algorithm is proposed for that setting. URI: http://hdl.handle.net/123456789/5781 Files in this item: 1
DanijelAleksicPhDThesis.pdf ( 4.605Mb ) -
Bogdanović, Katarina (Beograd , 2025)[more][less]
Abstract: In the rst and the second chapter of dissertation we prove some new inequalities for the spectral radius, essential spectral radius, oper- ator norm, measure of non-compactness and numerical radius of Hadamard (Schur) weighted geometric means of positive kernel operators on Banach function and sequence spaces. The list of extensions and re nings of known inequalities has been expanded. Some new inequalities and equalities for the generalized and the joint spectral radius and their essential versions of Hadamard (Schur) geometric means of bounded sets of positive kernel op- erators on Banach function spaces have been proved. There are additional results in case of non-negative matrices that de ne operators on Banach sequence spaces. In the third part we present some inequalities for opera- tor monotone functions and (co)hyponormal operators and give relations of Schur multipliers to derivation like inequalities for operators. In particular, let Ψ, Φ be s.n. functions, p ⩾ 2 and φ be an operator monotone function on [0, ∞) such that φ(0) = 0. If A, B, X ∈ B(H) and A and B are strictly ac- cretive such that AX−XB ∈ CΨ(H), then also AXφ(B)−φ(A)XB ∈ CΨ(H) and ||AXφ(B) − φ(A)XB||Ψ ⩽ r φ A+A∗ 2 − A+A∗ 2 φ′ A+A∗ 2 A+A∗ 2 −1 A(AX − XB)B B+B∗ 2 −1 r φ B+B∗ 2 − B+B∗ 2 φ′ B+B∗ 2 Ψ . under any of the following conditions: (a) Both A and B are normal, (b) A is cohyponormal, B is hyponormal and at least one of them is normal, and Ψ := Φ(p)∗ , (c) A is cohyponormal, B is hyponormal and ||.||Ψ is the trace norm ||.||1. Alternative inequalities for ||.||Ψ(p) norms are also obtained. URI: http://hdl.handle.net/123456789/5780 Files in this item: 1
Katarina_Bogdanovic_disertacija.pdf ( 1.621Mb ) -
Krstić, Mihailo (Beograd , 2025)[more][less]
Abstract: This doctoral dissertation addresses the integration of functions taking values in spaces of bounded operators and in spaces of complex measures on a given σ-algebra. The mentioned integrability is considered in a more general sense than that required in the theory of weak integration of vector-valued functions. The first part of the dissertation deals with the integrability of families of operators. If (Ω, M, μ) is a space with a positive measure μ and (At)t∈Ω is a family of operators from B(X, Y ), where X and Y are Banach spaces, then μ-integrability of the function Ω ∋ t 7 → ⟨Atx, y∗⟩ ∈ C is required for every x ∈ X and y∗ ∈ Y ∗. In this case, we prove that the quantity sup∥x∥=∥y∗∥=1 R Ω ⟨Atx, y∗⟩ dμ(t) is finite. This expres- sion allows us to define a norm on the corresponding vector space of families of operators. Furthermore, for every E ∈ M, one obtains an operator R E At dμ(t) in B(X, Y ∗∗), whose defining property is ⟨y∗, R E At dμ(t) x⟩ = R E ⟨Atx, y∗⟩ dμ(t) for every x ∈ X and y∗ ∈ Y ∗. The second part of the dissertation deals with the integrability of families of measures. If (λx)x∈X is a family of complex measures on (Y, A), where (X, B, μ) is a space with a positive measure μ, and if for every A ∈ A the function X ∋ x 7 → λx(A) ∈ C is μ-integrable, then the quantity supA∈A R X |λx(A)| dμ(x) is finite. This allows us to define a norm on the corresponding vector space of families of measures. In this case, for every B ∈ B there exists a complex measure R B λx dμ(x) on A such that R B λx dμ(x) (A) = R B λx(A) dμ(x) for every A ∈ A. The dis- sertation is organized as follows. The first part (Chapters 2–4) deals with the integration of functions taking values in B(X, Y ). Chapter 2 provides a survey of the known results on the integration of functions in B(H), where H is a separable Hilbert space, and presents original results extending the existing theory. In Chapter 3, the developed theory is applied to the Laplace transform of B(H)-valued functions, which has been previously considered in the literature. Chapter 4 is significant because it generalizes the integrability of functions taking values in B(X, Y ). This type of integration was first defined in [8]. The second part of the dissertation (Chapter 5) deals with the integration of functions taking values in spaces of complex measures on a given σ-algebra. The introduced type of integration is more general than Pettis concept and has been considered in [6, 7]. These works represent a natural ex- tension and application of the experiences gained from working with functions taking values in operator spaces, including original results of the candidate with coauthors. Numerous concrete examples are included, making this abstract material much more illustrative. URI: http://hdl.handle.net/123456789/5779 Files in this item: 1
Disertacija_M_Krstic.pdf ( 3.184Mb ) -
Milović, Matija (Beograd , 2025)[more][less]
Abstract: The subject of this dissertation is the study of the belonging of weak operator in- tegrals in appropriate ideals of compact operators, as well as the investigation of perturbation inequalities. These questions were previously considered in [16], where Cauchy–Schwarz type inequalities were established. In addition to providing norm estimates, these inequalities also yield sufficient conditions for an operator integral to belong to a given ideal. In the first part of the dissertation, using these inequalities, perturbation norm inequalities are derived for elementary operators generated by analytic functions. Specially, for an analytic function f, trigonometric polynomials T, S : R → C and t ∈ R, if fT S,t, f¯T T,t and f¯SS,t are the associated analytic functions, and if X ∈ B(H) and the operator P∞ n=1(AnXBn − CnXDn) belongs to a symmetric norming (s.n.) ideal CΦ(H), for some s.n. function Φ, then the following inequality holds ∞X n=1 (A∗ nAn− C∗ nCn) 1 2 fT S,t ∞X n=1 An⊗Bn X − fT S,t ∞X n=1 Cn⊗Dn X ∞X n=1 (BnB∗ n − DnD∗ n) 1 2 Φ ⩽ f¯T T,t ∞X n=1 A∗ nAn − f¯T T,t ∞X n=1 C∗ nCn 1 2 ∞X n=1 (AnXBn − CnXDn) × f¯SS,t ∞X n=1 BnB∗ n − f¯SS,t ∞X n=1 DnD∗ n 1 2 Φ , under certain conditions on the families (An)∞ n=1, (Bn)∞ n=1, (Cn)∞ n=1 and (Dn)∞ n=1 in B(H). Next, the dissertation considers vector measures induced by weak∗ integrable operator- valued functions taking values in Shatten–von Neumann ideals. Furthermore, the criteria for the compactness and nuclearity of the Gel’fand integral are derived, with emphasis on positive operator-valued functions. Finally, depending on the properties of the symmetric norming function Φ, the conse- quences of the condition sup e,f ∈B Z Ω Φ((⟨Aten, fn⟩)∞ n=1)dμ(t) < +∞. are explored. More precisely, it is proved that the weak∗ integral belongs to the symmetric ideal CΦ(H), as well as the Gelfand and Pettis integrability of the CΦ(H)-valued function A . URI: http://hdl.handle.net/123456789/5768 Files in this item: 1
Matija_Milovic_doktorska_disertacija.pdf ( 2.652Mb )