23. Vector Product(Cross Product) $ \vec{C}=\vec{A}\times \vec{B} $ $ C_{i}=\sum_{j,k}^{}\epsilon_{ijk}A_{j}B_{k} $ Levi-Civita Density(Permutation Symbol): $ \epsilon_{ijk}=\left\{\begin{matrix} 0 \\ +1 \\ -1 \end{matrix}\right. $ 0: two or three indexes are same ($ \epsilon_{112},\epsilon_{113},\epsilon_{121},\epsilon_{131},\epsilon_{211},\epsilon_{311} $) ($ \epsilon_{221},\epsilon_{223},\eps..