Posted by haifeng on 2021-07-01 00:02:15 last update 2021-07-01 00:02:38 | Answers (0) | 收藏
令
\[ f_n=\begin{cases} \chi_{_E}\ , & n\ \text{是奇数},\\ 1-\chi_{_E}\ , & n\ \text{是偶数}.\\ \end{cases} \]