**1. Introduction**

The economy is a human activity where interactions are particularly important. The mutual impacts are caused by an exchange of goods, services, and co-operation, but also competition, company overtaking, industrial espionage, etc. In result, one can observe a grouping among entities in the form of co-operation, branches, common interest, or competition on the market. These effects are the subject of many research fields, e.g., portfolio analysis [1–4], market structure analysis [5,6], globalisation researches [7–9], and many others.

The main tool for exploring the nature of interdependence among entities (companies, branches, shares, countries, etc.) is the cross-correlation analysis. In fact, this term is gathering a grea<sup>t</sup> variety of methods. Just mentioning the most often used: classical variance analysis and Pearson correlation coefficient [10–15], cointegration analysis [16–19], multifractal analysis [20–23], random matrix theory [24–27], power law classification scheme [28–30], or entropy-based methods [31,32].

The range of problems investigated by cross-correlation analysis is very broad, starting from sociology, economy, econophysics [20,23,33,34], transport [35,36], genome analysis, biology, food network, biochemistry network, science collaboration network [37], up to sport [38], and many others.

Within this study, the globalisation is analysed by the power law classification scheme (PLCS). In difference to other cross-correlation methods, such as detrended fluctuation analysis (DFA) [39–45] or the Pearson coefficient-based method [12,15,46,47], which are focused on noise correlation, PLCS is focused on trends. In the case of globalisation, trends seem to be more important, because they reflect similarities in evolution rather than mutual dependence and sensitivity to external impulses. Besides that, PLCS analysis allows for observing different features—medium-range correlations. On the other hand, the method is sensitive to long-term deterministic correlations that are related to "fundamental" effects [48]. The research analyses the currency exchange rate time series as an objective measure of mutual relationship and interactions among economies. The currency exchange rates are one of the most important parameters of the economy status. There are several

**Citation:** Mi´skiewicz, J. Network Analysis of Cross-Correlations on Forex Market during Crises. Globalisation on Forex Market. *Entropy* **2021**, *23*, 352. https:// doi.org/10.3390/e23030352

Academic Editor: H. Eugene Stanley

Received: 11 February 2021 Accepted: 11 March 2021 Published: 15 March 2021

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platforms where the exchange of currencies occur. The best known and one of the most important from the global point of view is the Forex market, which is focused on the institutional market. Besides that, there are many other exchange platforms that are aimed at individuals, such as exchange office, banks, and Internet exchange systems. The present study focuses on the Forex exchange time series, since the main goal is the analysis of economy globalisation, particularly cluster formation during stock market crises.
