Вісник Київського національного університету імені Тараса Шевченка. Серія: фізико-математичні науки
Not a member yet
409 research outputs found
Sort by
The problem of decentralized decision making for modern cloud services
Get PDFThe organization of modern cloud services is based on theoretical results in logistics, operations research, supply chains, information transmission (transportation) networks, and on the practical achievements of the novel information and communication technologies. As all the inhabitants of the planet become regular users and at the same time creators of such services, the issues of decentralized decision making are becoming everyday problems. The paper presents the setup for the problem of such solutions by suppliers (providers) of cloud services and suggests a mathematical formulation of the corresponding optimization problem with resource constraints. It is a starting point for further mathematical elaboration of the new everyday problems.
Pages of the article in the issue: 32 - 38
Language of the article: Ukrainia
Bernoulli multi-armed bandit problem under delayed feedback
Get PDFOnline learning under delayed feedback has been recently gaining increasing attention. Learning with delays is more natural in most practical applications since the feedback from the environment is not immediate. For example, the response to a drug in clinical trials could take a while. In this paper, we study the multi-armed bandit problem with Bernoulli distribution in the environment with delays by evaluating the Explore-First algorithm. We obtain the upper bounds of the algorithm, the theoretical results are applied to develop the software framework for conducting numerical experiments.
Pages of the article in the issue: 20 - 26
Language of the article: Englis
Analysis of educational and professional programs in the specialty 122 Computer Science in terms of program learning outcomes
Get PDFThe article presents an analysis of the educational and professional program "Informatics" of the first (bachelor\u27s) level of higher education in the sphere of knowledge 12 "Information Technology", specialty 122 "Computer Science", implemented at the Faculty of Computer Science and Cybernetics, Taras National University of Kyiv Shevchenko with educational and professional programs of the same level and specialty of other institutions of higher education of Ukraine in terms of program results.
During the analysis, they were compared with the approved standard of the first (bachelor\u27s) level of higher education in the specialty 122 "Computer Science".
In order to analyze the authors developed a database of educational programs. The ratio of program results in different programs by common specialty is analyzed.
Pages of the article in the issue: 89 - 101
Language of the article: Ukrainia
Fast calculations of Jackknife covariance matrix estimator
Get PDFWe consider data in which each observed subject belongs to one of different subpopulations (components). The true number of component which a subject belongs to is unknown, but the researcher knows the probabilities that a subject belongs to a given component (concentration of the component in the mixture). The concentrations are different for different observations. So the distribution of the observed data is a mixture of components’ distributions with varying concentrations. A set of variables is observed for each subject. Dependence between these variables is described by a nonlinear regression model. The coefficients of this model are different for different components. Normality of estimator for nonlinear regression parameters is demonstrated under general assumptions. A mixture of logistic regression models with continuous response is considered as an example. In the paper we construct confidence ellipsoids for the regression parameters based on the modified least squares estimators. The covariances of these estimators are estimated by the multiple modifications of jackknife technique. Performance of the obtained confidence ellipsoids is assessed by simulations.
Pages of the article in the issue: 27 - 36
Language of the article: Ukrainia
Modelling of contact interaction by “third body” in tribological problems
Get PDFThe variety of factors affecting the thermal conditions of a frictional couple results in complexity of the simulation of this problem. Among these factors, thermophysical (thermal conductivity, specific heat, coefficient of linear expansion of bodies, etc.) and also mechanical ones (elasticity, hardness of contacting bodies, etc.) play an important role. The conditions of friction, wear and heat generation are also determined by the characteristics of the so-called “third body”, i.e., thin near-surface and intermediate layers, the physical and mechanical properties of which differ from those of the interacting bodies, and by the microgeometry of their surfaces in the contact zone. The method of determination of thermal contact conductance in mathematical modelling of contact interaction with considering friction and hear generation by “third body” is presented. Using of modified conditions of heat contact in mathematical model of contact thermoelasticity taking into account of friction and heat generation is proposed. After numerical analysis, the graphs of dependence of thermal contact conductance on the input parameters are constructed and substantial influence of some of them is detected. On the example of one-dimensional non-stationary contact problem of thermoelasticity with heat generation of friction the influence of some physical and mechanical parameters on the temperature and heat fluxes in the contact bodies is investigated.
Pages of the article in the issue: 85 - 88
Language of the article: Ukrainia
Physico-mechanical characteristics of nanocomposite materials based on fluoroplastic
Get PDFIn this work, the elastic and strength properties of polymer composites with a polytrifluorochlorethylene matrix and a thermally expanded graphite filler were evaluated by the nanoindentation method, and the effect of the dispersion and concentration of the filler on the mechanical characteristics of nanocomposite materials was considered. It is shown that the microhardness of nanocomposite materials decreases with increasing filler concentration. An increase in the microhardness and Young\u27s modulus with the growth of filler particles was observed for nanocomposite materials with a filler concentration of 3 vol.%. As for nanocomposite materials with a filler volume fraction of 10 vol.%, similar behavior of the mechanical characteristics was observed with an increase in the dispersion of thermally expanded graphite particles to 180 ?m. A subsequent increase in the average particle size of thermally expanded graphite leads to a decrease in Young\u27s modulus and microhardness of composites, which may be associated with different degrees of polymer structuring and the formation of different orientational orders from the filler. The observed changes in Young\u27s modulus and H/E ratio indicate a change in the amorphous crystalline behavior of the composite to that characteristic of fine-crystalline materials and depends on the dispersion of thermally expanded graphite.
Pages of the article in the issue: 107 - 110
Language of the article: Ukrainia
Necessary conditions for solutions for the mixed type equations
Get PDFThe presented work consists of investigations of necessary conditions for linear mixed type equations in the limited area on a plane. These necessary conditions are determined by the help of integral relations and at the same time the fundamental solutions of such equations are used.
Pages of the article in the issue: 103 - 108
Language of the article: Englis
Mathetical problem of banking assets diversification
Get PDFIn article we consider a problem of optimal investment strategy by a commercial bank building. This task is actual and the development of a procedure to solve it can help in making investment banking decisions. The general formulation of the problem consists of two criteria. The first one is to maximize the expected return, and the second is to minimize the risk of the investment transaction. Mathematical formulation of the problem is considered as a problem of nonlinear programming under constraints. The procedure for solving such a two-criteria optimization problem allows to obtain many solutions, which requires further steps to make a single optimal solution. According to the algorithm proposed in the work, the problem is divided into two separate problems of single-criteria optimization. Each of these tasks allows to obtain the optimal values of the investment vector both in terms of its expected return and in terms of investment risk. Additional constraints in the mathematical formulation of the problem, make it possible to take into account factors that, from the point of view of the investor, may influence decision-making. The procedures presented in this work allow to obtain analytical representations of formulas that describe the optimal values of the investment distribution vector for both mathematical formulations of the problem.
Pages of the article in the issue: 85 - 88
Language of the article: Ukrainia
Application of fuzzy logic in finding the optimal filter in optoacoustics problems
Get PDFDenoising is an important step in the early stage of signal preprocessing in optoacoustic applications. The efficiency of such modern noise removal methods as wavelet or curvlet filtering depends significantly on the numerical combinations and forms of wavelet transform parameters, and the multidimensional extension of such filters is rather non-trivial. These issues are serious obstacle for using of these highly effective filters in the tasks of optoacoustic reconstruction, especially in real laboratorial or medical practice. The objective of our study was to find the optimal filter, convenient for use in laboratorian and medical practice, when the types of noise are a priori unknown, and the filter settings should not take much time. In the offered work spatial filters which have only one parameter of adjustment - the size of a window are considered. Three-dimensional extensions of such well-established denoising techniques, as mean filter, median filter, their adaptive variants (Wiener spatial filter and modified median filter), as well as iterative truncated arithmetic mean filter were analyzed. The proposed filters were tested on a test set that contains versions of Shepp-Logan\u27s three-dimensional phantom with mixtures of Gaussian and alpha-stable noise, as well as speckle noise. The identification of the best filter for simultaneous suppression of these types of interference was carried out using the theory of fuzzy sets. In our tests, a modified median filter and an iterative truncated arithmetic mean filter were rated as the best choice when the goal is to minimize aberrations when noise is not known a priory.
Pages of the article in the issue: 43 - 54
Language of the article: UkrainianDenoising is an important step in the early stage of signal preprocessing in optoacoustic applications. The efficiency of such modern noise removal methods as wavelet or curvlet filtering depends significantly on the numerical combinations and forms of wavelet transform parameters, and the multidimensional extension of such filters is rather non-trivial. These issues are serious obstacle for using of these highly effective filters in the tasks of optoacoustic reconstruction, especially in real laboratorial or medical practice. The objective of our study was to find the optimal filter, convenient for use in laboratorian and medical practice, when the types of noise are a priori unknown, and the filter settings should not take much time. In the offered work spatial filters which have only one parameter of adjustment - the size of a window are considered. Three-dimensional extensions of such well-established denoising techniques, as mean filter, median filter, their adaptive variants (Wiener spatial filter and modified median filter), as well as iterative truncated arithmetic mean filter were analyzed. The proposed filters were tested on a test set that contains versions of Shepp-Logan\u27s three-dimensional phantom with mixtures of Gaussian and alpha-stable noise, as well as speckle noise. The identification of the best filter for simultaneous suppression of these types of interference was carried out using the theory of fuzzy sets. In our tests, a modified median filter and an iterative truncated arithmetic mean filter were rated as the best choice when the goal is to minimize aberrations when noise is not known a priory.
Pages of the article in the issue: 43 - 54
Language of the article: Ukrainia
Self-organized Au-GaAs metal-semiconductor micro- and nanostructures for applications in plasmonic photovoltaics
Get PDFAu-GaAs metal-semiconductor composite microstructures have been prepared by an anisotropic etching of n-GaAs (100) wafers doped with Te (1016 to 1017 cm-3) with subsequent photostimulated chemical deposition of noble metal (Au) on formed semiconductor quasigratings. The microrelief topology of GaAs surface is controlled by the anisotropic etching conditions. Au metal was deposited on the structured GaAs surface as randomly placed nanoparticles of various shape and size and/or nanowires on the top of the hills of formed semiconductor microstructure. As the number of Au nanoparticles increases, they tend to localize on the ledges of the GaAs microrelief forming a system of approximately parallel nanowires. Obtained periodic structures with submicron to microns periods without Au nanoparticles and with deposited nanoparticles have been studied by means of scanning electron microscopy, optical spectroscopy (photoluminescence spectroscopy at room temperature), and photoelectric measurements. The decrease of the relative intensity of main photoluminescence band for samples with Au nanostructures compared to ones without nanoparticles deposition and simultaniously changes of the shape of photocurrent spectra of Au-GaAs microstructures have been observed. Such correlation in behaviour of measured spectra make formed Au-GaA metal-semiconductor microstructures perspective for the application in plasmonic photovoltaics.
Pages of the article in the issue: 139 - 142
Language of the article: Englis