tmp

\begin{equation}
\begin{aligned}
e_{ijk}e_{lmk} = \delta_{il}(\delta_{jm}\delta_{kk}-\delta_{jk}\delta_{km})-\delta_{jl}(\delta_{im}\delta_{kk}-\delta_{ik}\delta_{km})+\delta_{kl}(\delta_{im}\delta_{jk}-\delta_{ik}\delta_{jm})\
&\leq 3\delta_{il}\delta_{jm}-\delta_{il}\delta_{jm}-3\delta_{jl}\delta_{im}+\delta_{jl}\delta_{im}+\delta_{jl}\delta_{im}-\delta_{il}\delta_{jm}\
&\leq \delta_{il}\delta_{jm}-\delta_{jl}\delta_{im}
\end{aligned}
\end{equation}

来源：https://www.cnblogs.com/F-Mu/p/11695989.html