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The development of the mathematical basis of methods for model construction in various fields of science and technology today is very intensive and interdisciplinary. From the perspective of quantitative optimization methods, mathematical models of processes are of the utmost importance, whereby these can be generally represented in the form of linear algebraic equations or equivalent representations. Within the procedure of defining assumptions and adopting a process model, the problem formulation is conducted, whereby this refers to determining the level of problem decomposition and the level of detail in which the problem will be solved. The procedure of mathematical model determination, based on a process model, involves constructing a specific model that shows the relationships between the variables which describe the process, as well as the criterion of the solution effectiveness. The field of operations research that deals with this type of models is mathematical programming. Mathematical programming models represent a class of optimization models due to the fact that their goal is finding optimal solutions to problems. Analogously, the basic feature of today's modern economy is the penetration of mathematical methods into the essence of all economic research. In this context, the main aim of this paper is to point out the growing importance of the application of mathematical techniques, methods and models in economics as a science, as their proper application in economics can reduce many ambiguities in economic theory and practice.

This paper deals with the research of the risk of the profit and financial position of the energy industry in the period of Covid-19, i.e. sector D - Production and supply of electricity, gas and water subsector 351 - Production, transmission and distribution of electricity, 352 - Production of gas and distribution of gaseous fuels gas pipelines and 353 - Steam supply and air conditioning through selected indicators for the year 2020 compared to the previous year 2019, and evaluates them in relation to the average five-year achievements. Further analysis by subsectors revealed differences in the assessment of financial and profit positions by sub-sectors in relation to sectoral assessments. The results of the research show that, in the short term, no worsening of the risk factors of the profit and financial position was observed, based on the annual balance statements of the analyzed categories, at the level of the energy sector as a whole. Regression analysis also examined the influence of liquidity and solvency parameters on the rate of return on assets in the period before Covid-19 and during the Covid-19 pandemic.

The development processes which shaped the today's world market and international financial institutions have caused the need for financial statements standardization and harmonization at the global level. The International Accounting Standards Board (IASB) plays a crucial role in this process in the Republic of Serbia. By adopting the IFRS IASB project, IAS included, as the national standards, the Republic of Serbia started to apply a new system of financial reporting defined by legal and professional accounting regulations. The main purpose of this research is to indicate the cause-and-effect relationship between financial regulations and financial reporting formation according to the standards and harmonization requirements at the global level. With the aforementioned as the starting point, the main aim of the research in this paper is to indicate whether the financial statements based on the IASB project in the Republic of Serbia may be compared to the financial statements in other countries. The research results show that the immediate and exclusive IFRS implementation in the Republic of Serbia has become the limitation factor for the informational value of financial statements. Also, research results show that in addition to the IAS/IFRS, American accounting principles and the EU Directive 34 represent the key segments of professional regulations contributing to a better accounting harmonization, as well as a higher quality of financial reporting. Most countries with their own national financial regulations, harmonize their accounting rules with the above mentioned regulations when it comes to the most important issues.

Liquidity and profitability are closely related economic categories. The issue of constant balancing between liquidity and profitability, in theory known as "liquidity-profitability trade off", has been the subject of significant interest of the scientific community. There is no consensus on the direction of the impact of liquidity on profitability, but the existence of this impact in practice is confirmed. The aim of this paper is to investigate the impact of liquidity on profitability, based on selected traditional financial indicators, for medium and large enterprises in the group of processing and preserving of meat and meat products of the Republic of Serbia, in the period 2016 to 2019. The findings of a multiple linear regression analysis, show that the ratio of long-term sources and fixed assets in the group of processing and preserving of meat and meat products makes a statistically significant contribution predicting the return on assets.