Maya Stoyanova

Energy bounds for codes and designs in Hamming spaces

Designs, Codes and Cryptography, Oct 14, 2016

We apply linear programming techniques for obtaining lower and upper bounds on the energy of erro... more

Natural and mixed convection in a vertical water-flow chamber in the presence of solar radiation

Engineering Science and Technology, an International Journal, 2021

Natural and mixed convection in a vertical plane channel in the presence of solar radiation are a... more Natural and mixed convection in a vertical plane channel in the presence of solar radiation are analytically studied. The physical problem under consideration simulates water flow and heat transfer in a chamber with transparent rigid boundaries exposed to solar radiation. The intensity of the volumetric heat source across the channel declines exponentially as described by the Beer-Lambert law for light absorption in near-infrared and visible spectra. The flows are assumed laminar and fully-developed. The properties of the liquid are considered constant except for the variation of density defining the Boussinesq term in the momentum equation. For both natural and mixed convection it is possible to observe one, two or no reverse flows and the conditions for the existence of reverse flows are derived. Heat fluxes across the chamber as well as the power outlet and the bulk liquid temperature are calculated for values of the Reynolds number between zero and 500 and the Prandtl number for water. It is shown that the solar heat source in combination with the liquid heating from the chamber walls has a pronounced effect on the flow behavior and the heat transfer.

Impact of Motivation, Gamification and Learning Style on Students’ Interest in Maths Classes – A Study in 11 High School Grade

Advances in Intelligent Systems and Computing, 2018

In the age of dynamic development of information and communication technologies, students get bor... more In the age of dynamic development of information and communication technologies, students get bored quickly and easily lose their interest and motivation for learning when the taught material gets presented in the traditional ways. The purpose of the following study is to determine the level of impact of gamification on the process of studying Mathematics, and on the emotional condition of students with certain motivation and learning styles. The results of the study show that the evaluation and implementation of the gamification, achieved with the usage of Kahoot! in the process of studying Mathematics, do not affect the academic achievements of students with low intrinsic motivation in terms of studying mathematics, however, positively affect their emotional state, stimulate their interest and promote active learning.

Aluminium diffusion in titanium nitride films. Efficiency of TiN barrier layers

Applied Physics A Solids and Surfaces, 1993

Two kinds of reactively evaporated titanium nitride films with columnar (t30 films) and fine-grai... more Two kinds of reactively evaporated titanium nitride films with columnar (t30 films) and fine-grained (/3+ films) film structures, respectively, have been examined as diffusion barriers for preventing aluminium diffusion. The aluminium diffusion profiles have been investigated by 2 MeV 4He+ Rutherford backscattering spectrometry (RBS) at temperatures up to 550 ° C. The diffusivity from 300°C to 550°C is: D[m2s-x] = 3 x lO-18exp[-30/(RT)] in/30 layers and D[mZs-1] = 1.4 x 10-16 exp[-48/(RT)] in/3+

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Low-temperature iron-modified cobalt oxide system

Applied Catalysis A: General, 2001

The cobalt oxide system, modified with Fe was prepared by a solution technique at a room temperat... more The cobalt oxide system, modified with Fe was prepared by a solution technique at a room temperature. The periodic method of precipitation with inverse order of feeding the precipitator to the system (Me 2+ → OH − + OCl −) is used, which favors the formation of amorphous and highly disperse oxide system with non-stationary surface properties. The fresh sample as well as the samples obtained by its thermal treatment at different temperatures were characterized by Mössbauer and Infrared spectral analysis, X-ray diffraction and chemical analyses. A comparative consideration of the composition and structure of individual and modified with Fe cobalt oxide system has been carried out in order to clarify the influence of Fe as a modifying additive on the activity and selectivity of the sample. The results obtained show that the inclusion of Fe leads to the appearance of new active sites in the fresh samples-Fe(III) ions in octahedral coordination and to increasing the mobility of the reactive oxygen. The synthesized oxide system shows suitable properties for catalytic oxidation of toxic organic and inorganic compounds in an aqueous medium.

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On polarization of spherical codes and designs

Cornell University - arXiv, Jul 15, 2022

In this article we investigate the N-point min-max and the max-min polarization problems on the s... more In this article we investigate the N-point min-max and the max-min polarization problems on the sphere for a large class of potentials in R n. We derive universal lower and upper bounds on the polarization of spherical designs of fixed dimension, strength, and cardinality. The bounds are universal in the sense that they are a convex combination of potential function evaluations with nodes and weights independent of the class of potentials. As a consequence of our lower bounds, we obtain the Fazekas-Levenshtein bounds on the covering radius of spherical designs. Utilizing the existence of spherical designs, our polarization bounds are extended to general configurations. As examples we completely solve the min-max polarization problem for 120 points on S 3 and show that the 600-cell is universally optimal for that problem. We also provide alternative methods for solving the max-min polarization problem when the number of points N does not exceed the dimension n and when N = n + 1. We further show that the cross-polytope has the best max-min polarization constant among all spherical 2-designs of N = 2n points for n = 2, 3, 4; for n ≥ 5, this statement is conditional on a well-known conjecture that the cross-polytope has the best covering radius. This max-min optimality is also established for all so-called centered codes.

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Universal Bounds on Energy of Codes and Designs in H (n, q)

Distance Distributions and Energy of Designs in Hamming Spaces

Serdica Journal of Computing, 2015

We obtain new combinatorial upper and lower bounds for the potential energy of designs in q-ary H... more We obtain new combinatorial upper and lower bounds for the potential energy of designs in q-ary Hamming space. Combined with results on reducing the number of all feasible distance distributions of such designs this gives reasonable good bounds. We compute and compare our lower bounds to recently obtained universal lower bounds. Some examples in the binary case are considered.

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Bounds for the Minimum Distance and Covering Radius of Orthogonal Arrays via Their Distance Distributions

We propose two methods for obtaining estimations on the minimum distance and 1 covering radius of... more We propose two methods for obtaining estimations on the minimum distance and 1 covering radius of orthogonal arrays. Both methods are based on knowledge about the (feasible) 2 sets of distance distributions of orthogonal arrays with given length, cardinality, factors and 3 strength. New bounds are presented either in analytic form and as products of an ongoing project 4 for computation and investigation of the possible distance distributions of orthogonal arrays with 5 parameters in doable ranges.

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Serdica Journal of Computing DISTANCE DISTRIBUTIONS AND ENERGY OF DESIGNS IN HAMMING SPACES

We obtain new combinatorial upper and lower bounds for the potential energy of designs in q-ary H... more We obtain new combinatorial upper and lower bounds for the potential energy of designs in q-ary Hamming space. Combined with results on reducing the number of all feasible distance distributions of such designs this gives reasonable good bounds. We compute and compare our lower bounds to recently obtained universal lower bounds. Some examples in the binary case are considered.

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On the Remainders Obtained in Finding the Greatest Common Divisor of Two Polynomials

Sequential pattern mining is an important subject in data mining with broad applications in many ... more Sequential pattern mining is an important subject in data mining with broad applications in many different areas. However, previous sequential mining algorithms mostly aimed to calculate the number of occurrences (the support) without regard to the degree of importance of different data items. In this paper, we propose to explore the search space of subsequences with normalized weights. We are not only interested in the number of occurrences of the sequences (supports of sequences), but also concerned about importance of sequences (weights). When generating subsequence candidates we use both the support and the weight of the candidates while maintaining the downward closure property of these patterns which allows to accelerate the process of candidate generation. * This work is sponsored by a research grant from Vietnam National University, Hanoi (QG.15.41).

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New nonexistence results for spherical 5-designs

We investigate the structure of spherical 5-designs of relatively small cardinalities. We obtain ... more We investigate the structure of spherical 5-designs of relatively small cardinalities. We obtain some bounds on the extreme inner products of such designs. As a result, in 42 cases we prove nonexistence of designs of corresponding parameters. Our approach can be applied for other strengths and cardinalities.

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Universal Lower Bounds for Potential Energy of Spherical Codes

Constructive Approximation, 2016

We derive and investigate lower bounds for the potential energy of finite spherical point sets (s... more We derive and investigate lower bounds for the potential energy of finite spherical point sets (spherical codes). Our bounds are optimal in the following sense -they cannot be improved by employing polynomials of the same or lower degrees in the Delsarte-Yudin method. However, improvements are sometimes possible and we provide a necessary and sufficient condition for the existence of such better bounds. All our bounds can be obtained in a unified manner that does not depend on the potential function, provided the potential is given by an absolutely monotone function of the inner product between pairs of points, and this is the reason for us to call them universal. We also establish a criterion for a given code of dimension n and cardinality N not to be LP-universally optimal, e.g. we show that two codes conjectured by Ballinger et al to be universally optimal are not LP-universally optimal.

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Some methods for investigations of spherical designs

On the Riesz energy of spherical designs

We show how polynomial techniques can be applied for obtaining upper and lower bounds on the Ries... more

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A method for proving nonexistence of spherical designs of odd strength and odd cardinality

Problems of Information Transmission, 2009

We combine polynomial techniques with some geometric arguments to obtain restrictions of the stru... more We combine polynomial techniques with some geometric arguments to obtain restrictions of the structure of spherical designs with fixed odd strength and odd cardinality. Our bounds for the extreme inner products of such designs allow us to prove nonexistence results in many cases. Applications are shown for 7-designs.

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Nonexistence results for spherical 7-designs

moi.math.bas.bg

► Lemma 1.(BBD) Let C⊂ Sn− 1 be a τ-design with odd τ= 2e− 1. Then for every point x∈ C we have t... more

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New nonexistence results for spherical designs

Advances in Mathematics of Communications, 2013

We investigate the structure of spherical 5-designs of relatively small cardinalities. We obtain ... more We investigate the structure of spherical 5-designs of relatively small cardinalities. We obtain some bounds on the extreme inner products of such designs. As a result, in 42 cases we prove nonexistence of designs of corresponding parameters. Our approach can be applied for other strengths and cardinalities.

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Energy bounds for codes and designs in Hamming spaces

Designs, Codes and Cryptography, 2016

We apply linear programming techniques for obtaining lower and upper bounds on the energy of erro... more

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External field energy problems on the sphere and minimal energy points separation

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