Formula_21676

\[\begin{gathered}\sum_{i \neq j}: A_{1} \ldots \underbrace{}_{i} \ldots A_{j} \ldots A_{n}: \equiv \\\equiv \sum_{i \neq j} A_{i} A_{j} \eta(i, j): A_{1} \ldots A_{i-1} A_{i+1} \ldots A_{j-1} A_{j+1} \ldots A_{n}:\end{gathered}\]