Browsing Doctoral Dissertations by Title
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Đurić, Slavko (Kragujevac , 1968)[more][less]
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Ćirić, Dušan (Belgrade , 1981)[more][less]
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Zejnullahu, Ramadan (Priština)[more][less]
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Pevac, Irena (Belgrade)[more][less]
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Ćiprijanović, Aleksandra (, 2016)[more][less]
Abstract: Studying cosmic rays is very important for better understanding of high energy physical processes, since particles accelerated in space can reach energies far above what we can produce in accelerators on Earth, at the moment. Processes that produce cosmic rays are still unexplained to some extent, and models that have been proposed are awaiting confirmation. Presence of cosmic rays can be measured by detecting different products of interactions of these high energy particles with the interstellar medium through which they propagate. This thesis deals with the accretion shock as sources of cosmic rays in clusters of galaxies, as well as cosmic rays accelerated in supernova remnants inside galaxies. No matter which of these mechanisms is being considered, cosmic rays will collide with atoms and ions in the interstellar medium, and produce, among other things, gamma rays, neutrinos, as well as light elements, of which we will discuss lithium. In the thesis we primarily develop models that describe gamma rays produced by cosmic rays accelerated in shocks that can appear in different processes. We first examine accretion of new gas onto already virialized structures (for example in galaxy clusters). For the first time, we include the change of gamma-ray pro- duction with time, through the history of the universe, that reflects the evolution of accretion shocks which appear during large scale structure formation. Therefore, the models developed in this thesis describe the gamma rays from large scale struc- tures more realistically, compared to models which have previously been developed and which use single redshift approximation for the gamma-ray origin. Models are used to derive the gamma-ray flux of all unresolved galaxy clusters. These mode- led gamma rays are then compared to the isotropic diffuse gamma-ray background, measured by telescope Fermi-LAT. This leads to the conclusion that these cosmic rays have non-negligible contribution to the isotropic diffuse gamma-ray background (depending on the normalization, they can even explain the whole isotropic diffuse gamma-ray background) and that this population of cosmic rays has to be taken into consideration in addition to other components that are thought to be major contributors, like for example, unresolved normal galaxies or blazars. In the thesis, models of gamma-ray production in accretion shocks are also com- pared to observations of high-energy neutrinos detected by IceCube detector. Neu- trinos are used to normalize gamma-ray models, from which we conclude that if the accretion shocks are predominantly strong, neutrino background is more limiting to the possible gamma-ray emissivity of these objects, compared to the gamma-ray background we first used. Study of neutrinos as products of cosmic-ray interactions is very important, since neutrinos interact weakly with other particles, and therefore keep all of the information about the time they were produced and about cosmic rays that produced them. One part of the thesis deals with the production of cosmic rays in supernova remnants, in particular, the case of the Small Magellanic Cloud, which was detected in gamma rays. In this galaxy we also have the first measurements of the lithium abundances in the interstellar gas outside of the Milky Way. Since gamma rays and lithium are produced through interactions of cosmic rays with the interstellar medium, their same origin can be used to estimate the production of lithium and gamma rays by any cosmic-ray population. We show that galactic cosmic rays, which are considered to be dominant population of cosmic rays in the Small Magellanic Cloud, can only explain a very small part of the observed abundance of lithium, if we assume that the entire present gamma-ray emissivity that we observe also originates from the interaction of galactic cosmic rays with gas within the galaxy. This conclusion is interesting, because it leads to the possible existence of other sources of lithium in the Small Magellanic Cloud. Also, using the fact that gamma rays and lithium share the same origin, we estimate how much can irregular dwarf galaxies contribute to the diffuse gamma-ray background. Study of several different products of cosmic-ray interactions with the interstellar medium (gamma rays, neutrinos and lithium) on smaller scales (within the galaxy), as well as on the largest scales (galaxy clusters), showed that in addition to the galactic cosmic rays accelerated in supernova remnants, other still hypothetical co- smic rays (produced for example during accretion of gas on largest scales, or tidal interactions of galaxies) can have a non-negligible contribution to the measurements. URI: http://hdl.handle.net/123456789/4758 Files in this item: 1
AleksandraCiprijanovic.pdf ( 29.40Mb ) -
Bertolino, Milorad (Belgrade)[more][less]
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Dajović, Vojin (Belgrade)[more][less]
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Vujičić Stanković, Staša (Beograd , 2016)[more][less]
Abstract: The basic goal of this doctoral thesis is a research into different techniques and models which are applied in information extraction, and providing an informatic support in processing of natural language texts from culinary and gastronomy domain. Information extraction is a subfield of computational linguistics which includes techniques for natural languages processing, in order to find relevant information, define their meaning and establish relations between them. A very special attention is given to ontology based information extraction. It consists of the following: recognition of instances of ontology concepts in non‐structured or semistructured texts written in natural language, reasoning over the identified instances based on the rules defined in the ontology, as well as recognition of instances and their use for instantiating the proper ontology concepts. The main result of thesis reflects in the presentation of a new model for ontology based information extraction. Besides solving tasks of information extraction, the new model includes not only upgrade of existing lexical resources and ontologies, but also creation of the new ones. Its application resulted in development of a system for extraction of information related to the culinary domain, but this new model can be used in other fields as well. Beside this, the food ontology has been developed, Serbian WordNet is extended for another 1.404 synsets from the culinary domain, while electronic dictionary of Serbian is enlarged with 1.248 entries. The significance of the model application comes from the fact that the new and enriched linguistic resources can be used in other systems for natural language processing. The opening chapter of the thesis elaborates the need of providing an informatic model for processing a huge linguistic corpus related to culinary and gastronomy domain, through methodologically precise and solid approach integrating pieces of information on the domain. Also, the formalization of the basic research subject, text in electronic form, has been presented. Further on, the chapter contains a description of the natural languages approximations introduced in order to enable modern information technologies to process texts written in natural languages, and it emphasizes the need to make the characterisation of the text language with corresponding corpus and sublanguage. Further on in the first chapter, the task of information extraction, and the models for informatic processing of non‐structured or semi‐structured texts, used by the computer to interpret the meaning that the author (not necessarily a human) has intended to give while writing the text, are defined. Additionally, this chapter contains the description of the methods used in information extraction field – methods based on rules and methods based on machine learning. Their advantages and shortcomings are listed, so as the reasons why in this thesis are used techniques based on linguistic knowledge. As a conclusion to the introduction chapter, a special attention is given to ontologies, WordNet, and the significance of its usage as ontology. The second chapter contains the presentation of the linguistic resources and tools exploited in this thesis. It describes morphological dictionaries and local grammars used for solving the problem of information extraction from texts written in Serbian. A review of information extraction systems is given subsequently. At the end of the second chapter, the stages in processing of Serbian written texts during the information extraction in the software systems Unitex and GATE are described. The main result of the thesis is presented in the third chapter. It is the model for solving the problem of information extraction by integrating linguistic resources and tools, which includes creation of a text corpus, definition of tasks for information extraction, establishment of finite state models for information extraction, and their application accordingly, iterative enlarging of electronic morphological dictionaries, enrichment and enhancement of WordNet, and creation of new ontologies. Each of these steps is described thoroughly. Even though the model was at first considered as a solution for problems in processing Serbian, it can be equally applied for processing texts written in other languages, with the development of suitable language resources accordingly. The implementation of the above explained steps is described in the fourth chapter, through a system for information extraction from the culinary texts written in Serbian. Then follows the description of a bond in the development and mutual complement of lexical resources through steps in creating domain corpus, identifying culinary lexica, expanding and upgrading of WordNet and electronic morphological dictionaries, and developing of domain ontologies – the food ontology, the approximate measure ontology, and the ontology of ingredients that can be used as mutual replacements in the culinary domain. This system, developed for information extraction, has served for creating an advanced search system which, based on a corpus of culinary texts, generates all possible answers to inquiries made by users. In the frame of this system is implemented a specific method which serves for creation of links between different recipes. This is used in case when the user reviews a text of a recipe and notices that in preparing description features some part which already had appeared in other recipe, but with additional or different explanation. Another contribution of this thesis is application of developed ontologies in tasks that convert approximate measures into standard measures, and establishment of similarities among the recipes. The similarity of the recipes is defined as similarity of texts which describe process of course preparation in accordance with a specific recipe. The last chapter contains final conclusions and directions for future research. URI: http://hdl.handle.net/123456789/4410 Files in this item: 1
teza_Stasa.pdf ( 10.38Mb ) -
Hajduković, Dimitrije (Belgrade)[more][less]
URI: http://hdl.handle.net/123456789/38 Files in this item: 1
phdDimitrijeHajdukovic.pdf ( 1.535Mb ) -
Mališić, Jovan (Belgrade)[more][less]
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Jovalekić, Milica (Beograd , 2022)[more][less]
Abstract: Let M be a maximum and let N be a minimum of the non-negative martingale X1, X2, . . . , Xn. It is well known, that if X1 = 1, then γ(‖M ‖1) ≤ E (Xn log Xn) and γ(‖N ‖1) ≤ E (Xn log Xn) , where γ(x) = x − 1 − log x, for all x > 0. In this thesis, we prove the analogue of this result in the case when 1 < p < ∞, by proving that δp (‖M ‖p p ) ≤ ‖Xn‖p and δp (‖N ‖p p ) ≤ ‖Xn‖p, where δp(x) = ( 1 − 1 p ) x 1 p + 1 p x 1 p −1, for all x > 0. We also obtain a probabilistic proof of the fact min ρ∈D(Qn) ∫ Qn dx1 . . . dxn ρ (x1, . . . , xn)p−1 ∏n j=1 xαj +1 j = n∏ j=1 ( p p − αj − 1 )p , where p > 1, αj < p − 1 for j = 1, . . . , n and D (Qn) is family of all densities on the n-dimensional unit cube Qn = (0, 1)n in Rn. This provides the proof of the multidimensional weighted Hardy inequality. Namely, if f : Rn + → (0, ∞) is a measurable function, p > 1 and αj < p − 1 for j = 1, . . . , n, then ∫ Rn + n∏ j=1 xαj j Hnf (x)p dx ≤ n∏ j=1 ( p p − αj − 1 )p ∫ Rn + n∏ j=1 xαj j f (x)p dx, where Hnf (x) = 1 x1 . . . xn ∫ x1 0 · · · ∫ xn 0 f (t) dt, is a multidimensional Hardy operator, x = (x1, . . . , xn) ∈ Rn +, t = (t1, . . . , tn) and dt = dt1 . . . dtn. Let B(t) be a standard planar Brownian motion and r(θ) be the length of the projection of B[0, 1] on the line generated by the unit vector eθ = (cos θ, sin θ), where 0 ≤ θ ≤ π. We nd the common distribution function F of the random variables r(θ). Namely, we prove that F(x) = 8 ∞∑ n=1 ( 1 x2 + 1 (2n − 1)2π2 ) exp ( − (2n − 1)2π2 2x2 ) , for every x > 0. As immediate consequence, lower bound for the expected diameter of the set B[0, 1], better than known, is obtained. Namely, it is known that Ed ≥ 1.601, where d is the diameter of the set B[0, 1]. In this thesis we show Ed ≥ 1.856. URI: http://hdl.handle.net/123456789/5323 Files in this item: 1
MilicaJovalekicDisertacija.pdf ( 1.796Mb ) -
Zlatanović, Milan (Niš , 2010)[more][less]
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Milošević, Stefan (Beograd , 2017)[more][less]
Abstract: In this paper we present some norm inequalities for certain elementary operators and inner product type transformers, specially for Schatten norms, if the families of operators generating those transforms consists of arbitrary operators, and Q norms if at least one of those families consists of mutually commuting normal operators. Among others, we present inequalities that are generalizing the inequality p IA AX p IB B 6 X AXB ; from [11, Th. 2.3], for normal contractions and arbitrary unitarily invariant norm, to the case of Schatten norms and arbitrary contractions, as well as Q norms if at least one of the contractions A or B is normal. Also, by applying norm inequalities for operator monotone and operator convex functions, some refined Cauchy - Schwarz operator inequalities, as well as Minkowski and Landau - Gruss norm inequalities for operators are obtained as well. URI: http://hdl.handle.net/123456789/4656 Files in this item: 1
Stefan_Milosevic_Disertacija.pdf ( 2.544Mb ) -
Kurepa, Đuro (Paris , 1935)[more][less]
Abstract: This thesis is the first systematic study of trees and ramified partially ordered sets and of their close relationship to linear orderings. It was the source of many crucial notions and problems in this area as, for example, the notions of Aronszajn and Souslin tree. The problem whether inaccessible cardinals have the tree property i. e., whether they satisfy the analogue of Koning’s infinity lemma is considered in this thesis for the first time. The thesis consists of Chapter I (the subchapters t1-t8), Chapter II (the subchapters t9-t12), and an appendix ("Complément"). In t8A11 trees are classified as "large", "étrioit" and "ambigu" according to their heights and widths. In the Theorem 5bis the following property is presented: the very thin and tall trees ("étrioit") always have cofinal branches i.e., chains intersecting every level. This result was a source of the problem whether the same fact is true about the class of slightly wider trees ("ambigu") i.e., the trees of height equal to some cardinal Θ and whose levels are now only assumed to be of size less than Θ. This is the problem known today as the problem whether Θ has the tree property. In t10.2 the important notion σE is defined, where E is a linearly (or partially) ordered set. Namely, σE is a tree of all nonempty bounded and well-ordered subsets of E with the end-extension as the tree ordering. The problem whether inaccessible cardinals have the tree property appeared in t10.3. In t10.4 two following problems are mentioned: whether every Aronszajn tree is a subtree of σQ, and if every two uniformly branching Aronszajn trees are isomorphic. A question related to previous one, whether there is a homogeneous Aronszajn tree is also mentioned. The property that every two countable infinitely branching trees of the same height are isomorphic is proved in t10.5 (Theorem 1). Appendix contains a proof that Souslin’s problem is equivalent to the statement that every uncountable tree contains an uncountable chain or untichain i.e., that three are no Souslin trees. URI: http://hdl.handle.net/123456789/326 Files in this item: 1
disert.pdf ( 7.243Mb ) -
Šovljanski, Mirjana (Belgrade)[more][less]
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Petrić, Zoran (Belgrade)[more][less]
Abstract: In this dissertation methods of the proof theory are used to investigate coherence in some categories. Moreover, it is shown what the categorical notion of coherence means in the categorial proof theory. The thesis consists of three chapters. MacLane’s results for monoidal categories and symmetric monoidal categories are extended in Chapter 1 of the dissertation to some other categories with multiplication: relevant categories, affine categories and symmetric monoidal categories. All the results are formulated in terms of natural transformations equipped with “grafs” (g-natural transformations). It is proved, as consequences of these results, that relevant categories, affine categories and symmetric monoidal categories have the coherence property. Moreover, using these results, some basic relations between the free categories of these classes of categories are presented in Chapter 2 of the dissertation. In Chapter 3, an extension of the notion of dinatural transfomation is introduced in order to give a criterion of preservation of dinaturality under composition. An example of an application is given by proving that all cartesian closed canonical categories transformations are dinatural. Finally, an alternative sequent system for a fragment of intuitionistic propositional logic is introduced as a device, and a cut-elimination procedure is established for this system. URI: http://hdl.handle.net/123456789/187 Files in this item: 1
phdZoranPetric.pdf ( 3.130Mb ) -
Rakočević, Vladimir (Belgrade , 1984)[more][less]
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Martinović, Miroslav (Belgrade)[more][less]
URI: http://hdl.handle.net/123456789/191 Files in this item: 1
phdMiroslavMartinovic.pdf ( 3.650Mb ) -
Martinović, Nemanja (Beograd , 2017)[more][less]
Abstract: Within this dissertation results from the analysis of formation, abundance and evolution of the dwarf galaxies from cosmological simulations will be presented. We will present results from first cosmological simulation which was performed and analyzed in its entirety in Serbia. From the obtained results it was shown how the large scale structure formed, primarily the formation of clusters of galaxies. Since the clusters of galaxies are dominant structures in the universe, which are relatively slowly forming in relation to the existence of the universe, using the Illustris cosmological simulation, with included hydrodynamics, it will be shown that a significant part of the clusters of galaxies are not in the state of dynamic equilibrium, as some authors claim, therefore their formation process continues. The obtained results will be used to divide the clusters of galaxies into three types: interacting clusters, clusters interacting with smaller groups (weakly interacting clusters), and clusters of galaxies that are not interacting. As the dwarf galaxies constitute largest number of galaxies in the universe, notably largest number in the clusters of galaxies, this raises the question how cluster’s interactions affect the structure and abundance of a diverse type of dwarf galaxies within them. After a brief review of today’s known types of dwarf galaxies, we will show that there is susceptibility of the abundance of dwarf irregular galaxies in relation to whether the parent cluster of galaxies is in interaction or not. As it is assumed that the dwarf irregular galaxies are predecessors for the formation of the most numerous types of dwarf galaxies, this could point to the dominant channel of delivering these galaxies to clusters of galaxies where they are expected to undergo morphological transformation. In the end, the discovery of two formation channels for compact elliptical galaxies in the clusters of galaxies will be presented. The formation of this type of dwarf galaxies has been the subject of discussion due to limitations in observations and their compact nature from which it could not be concluded which process leads to their formation. It has been shown here that they occur either by tidal stripping of large spiral galaxies after their spiraling into the cluster of galaxies or forming directly inside clusters from the clouds of gas which is most probably formed by the accretion of cold gas from the environment. URI: http://hdl.handle.net/123456789/4505 Files in this item: 1
nm_dis1.pdf ( 25.52Mb ) -
Berić, Mladen (Belgrade , 1912)[more][less]