Approximation of the relationship between OPEC basket oil prices and the total index in the time of Covid-19

The financial crisis caused by the Covid-19 pandemic can affect all types of financial markets, on the other hand, considering the effective role of the OPEC basket oil price as an influential factor in the economy of oil exporting countries, It is important to examine the impact of changes in OPEC basket oil prices on financial markets during the Covid-19 pandemic. In this research, cubic spline interpolation has been used to find a two-dimensional curve to examine the changes in OPEC basket oil prices and the total index of the Tehran Stock Exchange on a monthly basis during the years 1397-1400. By calculating the approximation curve parametrically and in terms of the time variable, it was found that during a period of approximately six months after the prevalence of Covid-19, First in a period of almost three and a half months, the total index of Tehran Stock Exchange has been a descending function and then an ascending function in relation to OPEC oil price changes.