Unusual Kronecker Symbol

Question

I am studying on an article about the Galerkin method and I found this symbole $\delta_{ij}^{km}$. I know the definition of the usual Kronecker Symbol which is : $$\delta_{ij}=\cases{1&if $i=j$\\0&else},$$ I can't understand what $\delta_{ij}^{km}$ means.

Answer 1

$$ \delta_{ij}^{km} = \frac12\left( \delta_i^k \delta_j^m - \delta_j^k \delta_i^m \right) $$ (I'm a bit unsure about the factor $\frac12$.)