\begin{equation}
\psi(c)  &= \dfrac{1}{q} \sum_{d \in F_{q}^*} \psi(d) \sum_{b\in F_{q}} \chi_{b}(c) \overline{\chi_{b}(c)}\\
         &= \dfrac{1}{q} \sum_{b \in F_{q}} \chi_{b}(c) \sum_{d\in F_{q}^*} \psi(d) \bar{\chi}_{b}(d)\\
         &= \dfrac{1}{q} \sum_{\chi} G(\psi,\bar{\chi}) \chi(c) \  \text{ for }c\in F_{q}^*.
\end{equation}

出现错误

Misplaced alignment tab character &. \psi(c) &

Misplaced alignment tab character &. ^^I^^I &

Misplaced alignment tab character &. ^^I^^I &

当我去掉&的后最后排版效果没有换行

\begin{equation}
\psi(c)  = \dfrac{1}{q} \sum_{d \in F_{q}^*} \psi(d) \sum_{b\in F_{q}} \chi_{b}(c) \overline{\chi_{b}(c)}\\
            = \dfrac{1}{q} \sum_{b \in F_{q}} \chi_{b}(c) \sum_{d\in F_{q}^*} \psi(d) \bar{\chi}_{b}(d)\\
            = \dfrac{1}{q} \sum_{\chi} G(\psi,\bar{\chi}) \chi(c) \  \text{ for }c\in F_{q}^*.
\end{equation}