Index number rg(x_i) of a given array element x_i in a low-to-high sorted array rg(x):
| (1) | \{ x \} = \{ x_1, x_2, x_3 ... x_n \} \rightarrow \{ rg(x) \} = \{ x_{i_1}, x_{i_2}, x_{i_3} ... x_{i_n} \}, \, {\rm where} \, x_{i_1} \leq x_{i_2} \leq x_{i_3} ... \leq x_{i_n} |
so that rg(x_{i_k}) = k