AnmA^m_nAnm 像这种效果要怎么打出来呢?在两旁打上“Shift+4(英文)”中间写"A^n_m"
CnmC^m_nCnm 同理
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特别注意:要打An−1m+1A^{m+1}_{n-1}An−1m+1 的时候要加上大括号:{n - 1}