Dummy index
= Dummy index
A dummy index is summed over its full specified range. Its name can be changed consistently without changing the expression, as in $A_{ij}B_{ij}=A_{ji}B_{ji}$. A free index specifies a component and cannot be renamed in only one term of an equation. Relabelling dummy indices is useful for <tensor contraction> arguments involving a <symmetric tensor> and an <antisymmetric tensor>.