Methods for analyzing weakly correlated electrons were well established in the 20th century (Fig.9-6(a), left: blue circles are electrons) semiconductors, etc. It is hard to analyze strongly correlated electrons in heavy-element compounds (Fig.9-6(a), right: light blue circles indicate electrons, red indicates interactions). By extending the equations of quantum mechanics and allowing imaginary numbers to appear in the coefficients of the equations, the complex motion can be accurately captured. As a result, we can theoretically predict the appearance of a “Fermi arc” in the energy behavior of electrons (Fig.9-6(b)).