Note 1 to entry: The squared four-magnitude $S^2(\underset{\_}{\underset{\_}{x}},\underset{\_}{\underset{\_}{x}})=\underset{\_}{\underset{\_}{x}}\cdot \underset{\_}{\underset{\_}{x}}={\displaystyle {\sum }_{\mu \nu }{g}^{\mu \nu }{x}_{\mu }{x}_{\nu }}$ is the generalization of the squared magnitude of a three-dimensional vector. The scalar $S^2(\underset{\_}{\underset{\_}{x}},\underset{\_}{\underset{\_}{x}})$ can be positive, zero, or negative; it can be zero even in the case when $\underset{\_}{\underset{\_}{x}}\ne \underset{\_}{\underset{\_}{0}}$.