$$wdw\text{[n] =}｛\begin{array}{cc}\sqrt{\frac{{\displaystyle {\sum }_{我＝1}^{n}w［我］}}{{\displaystyle {\sum }_{我＝1}^{\raisebox{1ex}{$N$}\!\left/ \!\raisebox{-1ex}{$2$}\right.+1}w［我］}}}& \text{如果}1\le n<（\raisebox{1ex}{$N$}\!\left/ \!\raisebox{-1ex}{$2$}\right.）\\ \sqrt{\frac{{\displaystyle {\sum }_{我＝1}^{N-n}w［我］}}{{\displaystyle {\sum }_{我＝1}^{\raisebox{1ex}{$N$}\!\left/ \!\raisebox{-1ex}{$2$}\right.+1}w［我］}}}& \text{如果}（\raisebox{1ex}{$N$}\!\left/ \!\raisebox{-1ex}{$2$}\right.+1）\le n<N\end{array}$$