Browsing Doctoral Dissertations by Title

Spasić, Slađana (Belgrade)[more][less]

Cvetković, Ljiljana (Novi Sad)[more][less]

Janikijevik, Ljiljana (Skopje , 1973)[more][less]
URI: http://hdl.handle.net/123456789/157 Files in this item: 1
phdLjiljanaJanikijevik.pdf ( 4.625Mb ) 
Teorija onfinitezimalnih transformacija i njihova primena na integraljenje diferencijalnih jednačinaOkiljević, Blažo (Belgrade , 1986)[more][less]

Jarić, Jovo (Belgrade , 1973)[more][less]

Đaja, Časlav (Belgrade , 1967)[more][less]

Onić, Dušan (Beograd , 2013)[more][less]
Abstract: The evolution of supernova remnants is linked to the propagation of a collisionless shock wave, formed during the initial expansion of highvelocity supernova ejecta through the interstellar environment. Theoretical studies of shock waves are very important for the analysis of supernova remnants, as well as processes in the interstellar medium in general. In this doctoral dissertation, some theoretical results based on the magnetohydrodynamical theory of shock waves are presented, with special emphasis on ideal radiative magnetohydrodynamics for the optically thick case. Particularly, solutions for the case when jump in adiabatic index and/or ratio of gas to total pressure is allowed, are discussed. The main hypothesis of this dissertation is that thermal bremsstrahlung radiation at radio continuum frequencies can provide a signi cant contribution in the case of several Galactic supernova remnants. This hypothesis can give a natural explanation for nearly concave up radio continuum spectra of several Galactic supernova remnants that are expanding in the environment with higher than average density. In this context, it is important to identify the existence of the possible indicators of ensemble of thermal electrons at su ciently low temperatures and su ciently high densities so that the thermal bremsstrahlung radiation linked to a particular remnant could be observed at radio continuum frequencies (vicinity, interaction or expansion through the molecular cloud, presence of the cooled thermal Xray electrons during the post SedovTaylor phases, detection of lowfrequency turnovers associated with thermal absorption linked to the remnant, detection in H , identi cation of radio recombination lines linked to the remnant, etc). The signi cant presence of thermal component could theoretically explain radiospectral indices less than 0.5 measured for several evolutionary older supernova remnants, (mainly of mixedmorphology class) that expand in the high density region. Actually, these smaller radiospectral indices, under the assumption of simple power law, would represent a natural manifestation of a signi cant fraction of thermal emission at radio continuum frequencies. However, present knowledge of the radio continuum spectra of Galactic supernova remnants is still not determined precisely enough for any de nite conclusions to be made about the inherent thermal radioemission from supernova remnants. A thorough analysis is only possible in the case of three Galactic supernova remnants (3C396, IC443, 3C391) for which the thermal contribution is determined despite high associated uncertainties. New observations in the near future will lay the groundwork for making rmer conclusions about the existence of the socalled radio thermally active supernova remnants. This dissertation highlights the importance of observations of supernova remnants in X and rays, and multiwavelength analysis is general. Besides, it suggests a possible detection of rays from supernova remnant 3C434.1 based on the observations made by Fermi. URI: http://hdl.handle.net/123456789/3051 Files in this item: 1
Onic_Dusan_disertacija.pdf ( 10.67Mb ) 
Kuzmanović, Dragoslav (Beograd , 1986)[more][less]

Golubović, Zoran (Belgrade , 1984)[more][less]

Nešić, Slobodan (Belgrade , 1980)[more][less]

Baralić, Đorđe (Beograd , 2013)[more][less]
Abstract: The main objects studied in this doctoral thesis are quasitoric manifolds and spaces arising as the images of polyhedral product functors. Quasitoric manifolds are particularly interesting as topological generalization of nonsingular toric varieties. They are a research topic of many mathematical disciplines including toric geometry, symplectic geometry, toric topology, algebraic geometry, algebraic topology, theory of convex polytopes, and topological combinatorics. These objects have already found numerous applications in mathematics and sciences and they continue to be intensively studied. In this thesis we put some emphasis on combinatorial methods, focusing on the interaction of the geometry of toric actions and combinatorics of simple polytopes. This connection of geometry and combinatorics is based on the fundamental observation that convex polytopes naturally arise as orbit spaces of toric actions on quasitoric manifolds. Our main original contributions in this thesis are related to classical topological questions about degrees of maps between manifolds as well as their embeddings and immersions into Euclidean spaces. We follow the general scheme characteristic for Algebraic Topology where a topological problem is reduced, often by nontrivial reductions, to a question of arithmetical, algebraic, or combinatorial nature. We believe that the novel applications of this scheme developed in the thesis, especially the new techniques and calculations, have a potential to be applied on other problems about quasitoric manifods. Here is a summary of the content of the thesis. For the reader’s convenience and for completeness, in the first three chapters we give an elementary exposition of the basic theory of simplicial complexes, convex polytopes, toric varieties and quasitoric manifolds. The emphasis is on the fundamental constructions and central results, however the combinatorial approach, utilized in the thesis, allows us present the theory in a direct and concrete way, with a minimum of topological prerequisites. The mapping degrees of maps between quasitoric manifolds are studied in Chapter 4 with a particular emphasis on quasitoric 4manifolds. Utilizing the technique pioneered by Haibao Duan and Shicheng Wang, which is based on the intersection form and the cohomology ring calculations, we demonstrate that a complete information about mapping degrees can be obtained in many concrete situations. The theorems and the corresponding criteria for the existence of mapping degrees are formulated in the language of elementary number theory. It is amusing that the question whether a number appears as a mapping degree between concrete 4manifolds is directly linked with classical results from number theory such as whether a number can be expressed as a sum of two or three squares, etc. This approach allows us to analyze many concrete 4manifolds, including CP2, CP2♯CP2, S2×S2, etc. In Chapter 5 we calculate the StiefelWhitney classes of some concrete quasitoric manifolds and their duals. This information is used to determine cohomological obstructions to embeddings and immersions of these manifolds in Euclidean spaces. As an initial observation we showed that the calculations are highly dependent on the action of torus. Indeed, there are examples of quasitoric manifolds over the same polytope which exhibit a very different behavior and different complexity of the associated characteristic classes. Focusing on the quasitoric manifolds over the ndimensional cube, we are able to produce quasitoric manifolds which are very complex in the sense that they almost attain the theoretical minimum dimension for their embedding or (totally skew) immersion in Euclidean spaces. The thesis ends with an appendix with an outline of the theory of group actions and equivariant topology. URI: http://hdl.handle.net/123456789/4232 Files in this item: 1
phdDjordjeBaralic.pdf ( 8.102Mb ) 
Andrijević, Dimitrije (Belgrade)[more][less]
URI: http://hdl.handle.net/123456789/37 Files in this item: 1
phdDimitrijeAndrijevic.pdf ( 3.211Mb ) 
Dimitrijević, Radoslav (Belgrade)[more][less]
URI: http://hdl.handle.net/123456789/312 Files in this item: 1
phdRadoslavDimitrijevic.pdf ( 11.78Mb ) 
Cvetković, Dragoljub (Belgrade , 1985)[more][less]
URI: http://hdl.handle.net/123456789/249 Files in this item: 1
phdDragoljubCvetkovic.PDF ( 18.28Mb ) 
Cvetković, Dragoljub (Beograd , 1985)[more][less]
URI: http://hdl.handle.net/123456789/4101 Files in this item: 1
Trajektorije_pramenova.PDF ( 8.497Mb ) 
Predić, Bogoljub (Belgrade , 1984)[more][less]

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 nonlocal thermodynamic equilibrium (NLTE), which, in turn requires selfconsistent 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 highresolution 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 nonLTE 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 nonlocal 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 GaussSeidel iteration and “Sweepbysweep” implicit lambda iteration, latter one being based on “Forthandback” 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). “Sweepbysweep” 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 GaussSeidel iteration on problems with periodic boundary conditions. Also, it turns out that “Sweepbysweep” 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 nonLTE problem (Generalization to other iterative schemes is given in appendix A). This method allows for the selfconsistent 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 sixvectors 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 selfemitting 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 selfemitting disks dramatically changes not only intensity, but also polarized spectral lines pro files. Interplay of nonLTE, multidimensional effects and rotation results in very complicated line profiles which are nontrivial 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 doublelobed 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]

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 populationbased 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 wellknown practical problems. Comparative analysis with the stateoftheart 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]