2. Ratings

In this article, N denotes an integer greater than or equal to 1, and ${ℝ}^N$ is the Euclidean N-dimensional space; in practice, we usually have N = 1, 2 or 3, but not necessarily: in relativity, N = 4, and we may have to work in phase spaces with N > 4. The elements of ${ℝ}^N$ are vectors x = (x ₁ , x ₂ , . . . , x _N ). It is convenient to note them as in the case where N = 1; for example, their Euclidean norm will be written with absolute value...