Browsing Doctoral Dissertations by Title
-
Milić, Ivan (, 2014)[more][less]
Abstract: Multidimensional radiative transfer is an essential ingredient of modern ap- proach to modeling of astrophysical objects. Realistic modeling calls for the as- sumption of non-local thermodynamic equilibrium (NLTE), which, in turn requires self-consistent solution of coupled equations of radiative transfer statistical equilib- rium. This approach allows us to compute emergent spectrum from a given model of the object, which is, in principle, a necessary step in interpretation of observational results. Thanks to the high-resolution and high signal to noise observations, it is often possible to measure not only intensity of the light but also its state of po- larization. For interpretation of such observations it is necessary to solve radiative transfer problem for polarized radiation. This thesis deals with non-LTE transfer of (generally polarized) radiation in two- dimensional media. Thesis can be divided in two parts. In the first part, we present a numerical method for the formal solution of the radiative transfer equation in 2D Cartesian coordinate system. This method allows us to explicitly account for the contribution of non-local source functions to the local specific intensity, and, hence, to the local scattering integral. The knowledge of these contributions is necessary for an iterative solution of coupled equations of radiative transfer and statistical equilibrium. Based on this formal solution we introduce two novel schemes for multidimensional NLTE radiative transfer which have so far been used only in 1D geometry: symmetric Gauss-Seidel iteration and “Sweep-by-sweep” implicit lambda iteration, latter one being based on “Forth-and-back” implicit lambda iteration. Both methods utilize implicit use of the local source function and the source func- tion corrections each sweep of the computational grid (four times per iteration). “Sweep-by-sweep” implicit lambda iteration also uses the idea of iteration factors and achieves acceleration of about factor of seven with respect to the referent Ja- cobi method. Both new methods also significantly surpass both Jacobi iteration and Gauss-Seidel iteration on problems with periodic boundary conditions. Also, it turns out that “Sweep-by-sweep” implicit lambda iteration scales with grid resolu- tion better than the Jacobi iteration. The second part of the thesis deals with numerical polarized radiative transfer on 2D cylindrical grids. The method is based on the second order short characteristics for the formal solution and uses standard Jacobi iteration with Ng acceleration to solve the polarized non-LTE problem (Generalization to other iterative schemes is given in appendix A). This method allows for the self-consistent solution of coupled equations of radiative transfer and statistical equilibrium equation for a two level atom model for polarized radiation. The method employs reduced intensity basis where intensity and source function are written as six-vectors and source function does not depend on direction which allows for significant saving in memory and computing time. It is applicable for modeling of axisymmetric objects such as as- trophysical disks. The method has been tested on simple models of circumstellar and self-emitting disks. The most important conclusion of these computations is that the presence of the disk in the system introduces a significant degree of linear polarization due to the scattering processes and that one is able to model it in detail using our approach. Also, it is shown that the presence of rotation in self-emitting disks dramatically changes not only intensity, but also polarized spectral lines pro- files. Interplay of non-LTE, multidimensional effects and rotation results in very complicated line profiles which are non-trivial for interpretation. However, the main effect is that the rotation decreases the amount of Stokes Q component and, de- pending on the rotation velocity causes appearance of double-lobed U polarization profile. If these effects can be observed, this kind of modeling provides a useful tool for interpretation of the spectropolarimetric observations. URI: http://hdl.handle.net/123456789/4757 Files in this item: 1
IvanMilic_teza.pdf ( 1.075Mb ) -
Popović, Nikola (Belgrade)[more][less]
-
Zeković, Vladimir (Beograd , 2020)[more][less]
Abstract: In the disertation, by the linear plasma theory we show that resonant electromagnetic (EM)micro-instabilities are excited by the two colliding plasmas which are interpenetrating eachother with the super-Alfv ́enic velocity, in the direction quasi-parallel to the magnetic field.The expected Rankine-Hugoniot shock conditions, naturally arise as a consequence of a highlyresonant interaction of ions with the instability. By using kinetic simulations, we find herethat such resonant instabilities appear in the linear stage, and we show how these instabilitiestrigger the shock formation during the non-linear stage.By theoretical modeling, we show how a magnetosonic soliton forms and leads to the periodicshock reformation and initiation of the return current of ions, which drives the EM upstreaminstability. We find that ions are being pre-accelerated by the upstream and shock instabilities ina mechanism that is similar to the diffusive shock acceleration (DSA). By our EM test particlesimulation runs we show that at quasi-parallel shocks, EM instabilities highly contribute toelectron pre-acceleration, leading to the formation of a power-law electron spectra through theFermi-like mechanism.By very long kinetic simulation runs, in this disertation we find that ions and electronsenter DSA in a similar number which is further applied in a model of non-linear DSA. Withinthis model, we obtain the theoretical particle spectra and we find the electron-to-proton ratioat high energies for the different Mach numbers. We show that the spectra of quasi-thermalparticles at the shock can be represented by a non-equilibriumκ-distribution. We find thatthe level of modification decreases andκ-index increases behind the shock, implying that theparticle distribution tends to become a Maxwellian. URI: http://hdl.handle.net/123456789/5094 Files in this item: 1
ZekovicVladimir.pdf ( 56.60Mb ) -
Bačanin Džakula, Nebojša (Beograd , 2015)[more][less]
Abstract: Hard optimization problems that cannot be solved within acceptable computational time by deterministic mathematical methods have been successfully solved in recent years by population-based stochastic metaheuristics, among which swarm intelligence algorithms represent a prominent class. This thesis investigates improvements of the swarm intelligence metaheuristics by hybridization. During analysis of the existing swarm intelligence metaheuristics in some cases de ciencies and weaknesses in the solution space search mechanisms were observed, primarily as a consequence of the mathematical model that simulates natural process as well as inappropriate balance between intensi cation and diversi cation. The thesis examines whether existing swarm intelligence algorithms for global optimization could be improved (in the sense of obtaining better results, faster convergence, better robustness) by hybridization with other algorithms. A number of hybridized swarm intelligence metaheuristics were developed and implemented. Considering the fact that good hybrids are not created as a random combination of individual functional elements and procedures from di erent algorithms, but rather established on comprehensive analysis of the functional principles of the algorithms that are used in the process of hybridization, development of the hybrid approaches was preceded by thorough research of advantages and disadvantages of each involved algorithm in order to determine the best combination that neutralizes disadvantages of one approach by incorporating the strengths of the other. Developed hybrid approaches were veri ed by testing on standard benchmark sets for global optimization, with and without constraints, as well as on well-known practical problems. Comparative analysis with the state-of-the-art algorithms from the literature demonstrated quality of the developed hybrids and con rmed the hypothesis that swarm intelligence algorithms can be successfully improved by hybridization. URI: http://hdl.handle.net/123456789/4245 Files in this item: 1
phdBacaninNebojsa.pdf ( 3.813Mb ) -
Lučić, Zoran (Belgrade , 1985)[more][less]
-
Madaras-Silađi, Rozalija (Novi Sad)[more][less]
URI: http://hdl.handle.net/123456789/193 Files in this item: 1
phdRozalijaMadarasSiladji.pdf ( 3.002Mb ) -
Vujošević, Luka (Belgrade , 1964)[more][less]
-
Jovanović, Milan (Belgrade)[more][less]
-
Lazarević, Ivan (Beograd , 2022)[more][less]
Abstract: In this doctoral thesis we obtained some results in graph theory and its applica tions. In the rst chapter, we give the review of basic notions and theorems of combinatorial theory of graphs, spectral theory of graphs, random graphs and distribution of their eigenvalues. The most detailed consideration is given to adjacency matrix and properties of its spectrum. In particular, in this dissertation we study Energy of graphs and generalizations of it. Energy of graph is the sum of absolute values of eigenvalues of a graph. Schatten norms of graphs represent p-th degree norm of singular values of graph, and the special cases of this norm for p = 1 corresponds to the Energy of graph. In chapter three of this dissertation we are given the most original scienti c contribution. We prove the conjecture of Nikiforov about Schatten norms of graphs when p > 2. First we prove that conjecture is true for some special classes of graph (for trees and strongly regular graph with maximal energy). After that, we prove the conjecture in the general case. Auxiliary theorem obtained in the proof of this conjecture is also an original result which gives a sharp upper bound of sum of quadratic of the largest k singular values of graph. A corollary of this theorem which gives an upper bound for sum of squares of the biggest two singular values of graph can be useful in further research. In the subsection 3.3 we give an original theorem about asymptotic properties of spectrum and thus energy of complement graph for a large values of n. In the subsection 3.4 we calculate a mean of p-th degree of singular values and upper bound of geometric mean of almost all graphs. The last chapter shows relation between combinatorial theory of graphs with universal universal algebra and mathematical logic. The central part of this chapter is original and shorter proof of an important theorem which solves a dichotomy conjecture for CSP problem on undirected graphs. URI: http://hdl.handle.net/123456789/5371 Files in this item: 1
Ivan_teza20042022.pdf ( 1.428Mb ) -
Alatrash, Emhimed Salem (Beograd , 2015)[more][less]
Abstract: Ontologies, often defined as an explicit specification of conceptualization, are necessary for knowledge representation and knowledge exchange. This means that ontology describes concepts and relations that exist in a domain. To enable knowledge exchange, it is necessary to describe these concepts and relations in a better way than just ordering them in taxonomy. A computational ontology consists of a number of different components, such as Concepts, Instances, Individuals or Facts, Relations and Attributes. The present research is intended to consider different software tools related to Semantic web, and achieve a kind of comparison among them. In fact, five ontology-editors are described and compared. They are: Apollo, Onto Studio, Protégé, Swoop and TopBraid Composer Free Edition. The structure and basic features of these editors as well as the way of using them are described. The main criterion used in the process of comparing these editors lies in their convenience for the user, and the possibility to apply them in different kinds of application. The main goal of the work is to introduce a method for ontology construction of a certain domain in applying the Semantic web. A number of software tools adapted to build up the domain ontologies of most wide–spread natural languages are available; however accomplishing that for any given natural language presents a challenge. This research proposes a semi-automatic procedure to create ontologies for different natural languages. The approach utilizes various software tools that are available on the Internet, most notably DODDLE-OWL which is a domain ontology development tool implemented for English and Japanese languages. Through this tool, WordNet, Protégé and XSLT transformations, the researcher proposes a general procedure to construct domain ontology for any natural language. URI: http://hdl.handle.net/123456789/4266 Files in this item: 1
phdEmhimedAlatrash.pdf ( 2.171Mb ) -
Jović, Aleksandar (Beograd , 2021)[more][less]
Abstract: The continuous-time programming problem consists in minimizing an integral functional, with phase constraints of different types. The subject of this doctoral dissertation is to establish extremum conditions as well as duality theorems for a class of convex and smooth continuous-time programming problems, with phase constraints of the inequality type. Unfortunately, some of the results in this field are not valid, which is confirmed in 2019. In this paper, new optimality conditions for the aforementioned class of problems are ob tained. The theorems of weak and strong duality are proved. The main tool for deriving these results is a new theorem of the alternative for a convex system of strict and nonstrict inequal ities in infinite dimensional spaces. In order to apply the aforementioned theorem, a suitable regularity condition must be satisfied. Some optimality conditions are obtained with additional constraint regularity qualification. Theoretical results are confirmed by practical examples. URI: http://hdl.handle.net/123456789/5298 Files in this item: 1
A.Jovic_doktorska_disertacija.pdf ( 1.280Mb ) -
Nikolić, Mladen (Belgrade , 2013)[more][less]
Abstract: In this thesis the problem of guiding search in automated theorem proving is considered. The thesis consists of two parts that have the CDCL search system, the system intensively used by modern SAT solvers, as their common topic. In the rst part of the thesis a simple approach to guiding search is considered | guiding by the selection of the solver, its heuristics, and their parameters, based on the properties of an instance to be solved. The basis of the proposed methods for algorithm selection is syntactical similarity of formulae which is re ected in their graph structure. This graph similarity is established and analyzed by using an original graph similarity measure (which turned out to be useful in other contexts, too). Yet, practical approaches to measuring similarity of formulae are based on their numerical features due to the computational complexity issues. Two simple methods for algorithm selection, based on k nearest neighbors, were proposed. The rst technique, ArgoSmArT is based on classi cation of instance in one of the prede ned families for which the e cient algorithms are known. The instance is solved by algorithm corresponding to the family to which the instance was classi ed. The second technique, ArgoSmArT k-NN is based on nding several similar instances in the training set for which the solving times by all considered algorithms are known. The instance is solved by the algorithm that behaves the best on those instances. ArgoSmArT technique is better suited for con guration selection of a SAT solver, and ArgoSmArT k-NN for SAT solver selection. ArgoSmArT k-NN technique showed to be more e cient than the most important and very complex system for SAT solver selection | SATzilla system. Apart from CNF SAT solver selection, the problem of non-CNF SAT solver selection is considered. The focus was not on solver selection techniques, since the proposed techniques are directly applicable, but on the attributes that can be used to describe non-CNF SAT instances, which have not been proposed earlier. The results in this domain are positive, but still limited. The main reason for that is the lack of greater number of non-CNF SAT solver of di erent behaviour, which is not surprising, having in mind that this kind of solvers is in its early stage of development. Apart from construction of e cient SAT solver selection system, the methodology of SAT solver comparison, based on statistical hypothesis testing is proposed. The need for such a methodology comes from great run time variations of single instance solving by a solver, which can result in di erent SAT solver orderings when one tries to compare their performance or rank them, as experimentally demonstrated. The proposed methodology gives the estimate of statistical signi cance of the performed test and the estimate of the e ect size, for instance the probability of a solver being faster than another. The second part of the thesis is concerned with generalizing CDCL search system to fragments of rst order logic. The proposed system can be used as a basis for e cient proving in some fragment if the rules of resolution and factoring are speci ed for that fragment. These rules are de ned for an extension of coherent logic. The soundness and completeness of the system are proved. The system has several distinguishing features which are a consequence of previously performed analysis of challenges in coherent logic theorem proving. The system enables rst order reasoning, instead of ground one characteristic for all existing coherent logic provers. Moreover, it introduces backjumps and lemma learning. The special attention in system design was paid to the possibility of generating readable proofs by the prover implementing the system. This possibility is one of the greatest qualities of coherent logic, but it is not easy to achieve if CDCL search system is used. One of the properties of the system that came from the need for generation of readable proofs is preservation of quanti ers in proving process which is rather unusual for existing CDCL systems. Another advantage of the proposed CDCL system is the possibility of transfer of heuristics which are already successfully employed in SAT solving to other domains. Based on the proposed system, the proof procedure Calypso for extended coherent logic was de ned which can also be used in standard coherent logic. The extension of Rete algorithm which enables detection of con icts and literals to be propagated or decided is proposed. Procedure Claypso is implemented in C++. It was evaluated on a representative coherent logic problems and it showed superior to other coherent logic provers and also the prover Vampire, the most e cient prover for rst order logic. Based on the results presented in this thesis, it can be concluded that the main hypothesis of this work, that the search system used in CDCL SAT solvers can be signi cantly improved by simple guiding and that it can be successfully formulated for fragments of rst order logic such as coherent logic, was con rmed and that the concrete answers on how to do that were provided. URI: http://hdl.handle.net/123456789/2584 Files in this item: 1
nikolic_mladen.pdf ( 1.448Mb ) -
Jovanović Spasojević, Tanja (Beograd , 2022)[more][less]
Abstract: In this thesis, subjects of consideration are the embeddings theorems of weighted Bergman spaces in Lp-spaces, as well as embeddings theorems of harmonic mixed norm spaces. The first part of the thesis generalizes the theorems of embeddings Bergman spaces into Lp(μ)-spaces, where μ is a Borel measure on a given domain. They have been earlier studied on domains such as unit ball and upper half-space. Generalization refers to bounded domains Ω ⊂ Rn with C1 boundary. This embedding will be valid to any p > 0, whenever the measure of the spaces Lp satisfies the Carledon condition. Reverse the direction will be valid only in case if p > 1 + α+2 n−2 . The second part of the dissertation also generalizes the embeddings theorems of mixed norm spaces of harmonic functions on a unit ball, where the generalization is applied to the domain Ω ⊂ Rn with C1 boundary. However, in addition we are obtaining another important result relating to the limitation of the maximum operators in the mixed norm on the general domain for the class of QNS functions. URI: http://hdl.handle.net/123456789/5378 Files in this item: 1
Jovanovic_Spasojevic_Tanja.pdf ( 1.643Mb ) -
Jovanović, Predrag (Belgrade , 2005)[more][less]
-
Dimovski, Igor (Novi Sad , 2011)[more][less]
Abstract: A comprehensive pedagogical research regarding teaching mathematics at a tertiary, university level has been presented in the PhD dissertation. The educational resources tailored in an electronic form using the programme package Matlab are integrated in the learning process. The impact of ICT use to the essential knowledge that refers to multivariate calculus (functions of several variables, vector-valued functions and the three-dimensional analytical geometry) has been statistically explored by intensive use of 3D static and dynamic visual tools. Part of the students who have participated in the research have developed Matlab programmes all by their own. One part of the research has been focused on probable impact of the programming skills on learning mathematical concepts. URI: http://hdl.handle.net/123456789/3874 Files in this item: 1
PhD_I_Dimovski.pdf ( 5.423Mb ) -
Pavlović, Marina (Beograd , 2022)[more][less]
Abstract: The study of galaxies through high redshifts are key to understanding the evolution of galaxies through cosmic times. As such objects are very difficult to observe directly, they are mainly examined using empirically derived tools such as the numerous correlations between their different parametric characteristics, one of them being the linear relationship between far-infrared and radio emission in star-forming galaxies, named the Far Infrared-Radio (FIR) Correlation.Although the correlation was considered to be stable in terms of linearity, recent works, which include galaxies at high redshifts (0 < z < 6), showed a large deviation from the correlation in these systems. The goal of this doctoral dissertation is an examination of the physical processes that lead to this kind of behavior. As a possible cause of this evolution, we will assume for the first time, and examine interactions between galaxies (collisions and close approaches). Interactions be- tween galaxies lead to the formation of shock waves on large scales that can lead to changes in the relationship between infrared and radio emissions. Our hypoth- esis was tested in several stadiums and the main results are as follows: 1. We developed models of the evolution of the FIR correlation with redshift as functions of the galaxy interaction rate. We tested the models on a sample of galaxies with an already determined morphology separately for disc galaxies and for galaxies that have recently been or are currently interacting - irregular galaxies. 2. In a small sample of 34 galaxies that we took from paper Miettinen et al. (2017), it was shown that there is an indication that the interaction between galaxies can be responsible for the evolution of the correlation with the redshift. 3. The next analysis was performed on a much larger sample of star-forming galaxies taken from COSMOS field, where we did not find any evolution of corre- lation with the redshift. Also, it was shown that the mean value of the correlation parameter is lower in irregular galaxies than in disk galaxies. Although recent observations indicated an evolution of the FIR correlation with redshift, the results of this research failed to reproduce that evolution and showed that the FIR correlation is stable with redshift. However, it was also shown that due to the interaction of galaxies, the evolution of the FIR correlation is possible if the representation of interacting systems in the sample is higher. URI: http://hdl.handle.net/123456789/5460 Files in this item: 1
Disertacija_Marina_Pavlovic.pdf ( 8.624Mb ) -
Milanović-Lazarević, Smilja (Belgrade , 1987)[more][less]
URI: http://hdl.handle.net/123456789/248 Files in this item: 1
phdSmiljaMilanovicLazarevic.PDF ( 4.155Mb ) -
Dimitrijević, Milan (Belgrade)[more][less]
Abstract: A detailed analysis of the motion of a charged particle under the influence of Coulomb, polarization, and quadrupolar potential has been performed, and analytical expressions for particle path parameters have been obtained. Also, conditions for existence of a critical impact parameter, which separates paths ending in the atom from those going to infinity, have been examined, and methods for its determination have been elaborated. Results of this investigations have been applied to the theory of Stark broadening (broadening of spectral lines under the influence of collisions with charged particles), where a special attention has been paid to the adiabatic limit, and to quasistatic and impact approximations. Modification of Stark broadening functions has been performed and it has been demonstrated that the biggest influence of path curvature on them, is in the vicinity of the critical impact parameter, which separates paths ending in the atom from those going to infinity. The change in the values of the Stark broadening functions will have the largest influence on Stark broadening parameters for lines for which, at the most probable velocity, influence of the contribution of inelastic collisions is dominant and effective polarizability large. The application of semiempirical approximative approach for calculation of Stark broadening parameters for multiply charged emitters, has been analyzed as well, and the modification of semiclassical theory has been performed. The averaged ratio of experimental and theoretical results is 0.75 for semiempirical, 1.47 for semiclassical, and 1.04 for modified semiclassical approach.У раду је извршена детаљна анализа кретања наелектрисане честице под утицајем Кулоновог, поларизационог и квадруполног потенцијала и добијени су аналитички изрази за параметре путање честице. Такође су испитани услови за егзистенцију критичног сударног параметра, који раздваја путање које завршавају у атому од оних које одлазе у бесконачност, и разрађене методе за његово налажење. Резултати овог испитивања су примењени на теорију Штарковог ширења (ширења спектралних линија под утицајем судара са наелектрисаним честицама), при чему је посебна пажња посвећена адијабатској граници и квазистатичкој и сударној апроксимацији. Изведена је модификација функција Штарковог ширења и показано је да је највећи утицај закривљености путање на њих, у близини критичног сударног параметра, који раздваја путање које завршавају у атому од оних које одлазе у бесконачност. Промена у величини функција Штарковог ширења ће највише утицати на параметре Штарковог ширења код линија за које је, при највероватнијој брзини, утицај доприноса нееластичних судара доминантан а ефективна поларизабилност велика. Такође је анализирана примена семиемпиријског апроксимативног прилаза за израчунавање параметара Штарковог ширења за вишеструко наелектрисане емитере, при чему је извршена модификација семикласичне теорије. Усредњени однос експерименталних и теоријских резултата је 0.75 за семиемпиријски, 1.47 за семикласични а 1.04 за модификовани семикласични прилаз. URI: http://hdl.handle.net/123456789/77 Files in this item: 1
Dimitrijevic.pdf ( 6.262Mb ) -
Kovjanić, Milorad (Beograd , 2023)[more][less]
URI: http://hdl.handle.net/123456789/5598 Files in this item: 1
Milorad Kovjanić - Disertacija.pdf ( 2.190Mb ) -
Vince, Ištvan (Belgrade)[more][less]