Narrow Results

In this paper, we analyze 26 Chinese sectoral indices and evaluate the effects of the crisis caused by COVID-19 on its efficiency. We calculated the degree of multifractality in the pre- and post-COVID-19 period and found that it increases, albeit unevenly, for the economic sectors. The results suggest that global crises can affect the efficiency of the stock markets in an unequal way, with important implications for portfolio management, risk management, financial regulation and the development of predictive models.

The study of strange nonchaotic attractors (SNAs) has been mainly restricted to quasiperiodically forced systems. At present, SNAs have also been uncovered in several periodically forced smooth systems with noise. In this work, we consider a periodically forced nonsmooth system and find that SNAs are created by a small amount of noise. SNAs can be generated in different periodic windows with weak noise perturbation. If the parameter is varied further from the chaotic range, a larger noise intensity is required to induce SNAs. Besides, noise-induced SNAs can be generated by the periodic attractors near the boundary crisis. In addition, with the increasing noise intensity, the intermittency between SNAs and periodic attractors can be induced by transient chaos. The characteristics of SNAs are analyzed by the Lyapunov exponent, power spectrum, singular continuous spectrum, spectral distribution functions, and finite time Lyapunov exponent.