Simplify delta(ij)delta(ij). The (ij) parts should be read as subscripts to the delta.
Can anyone give me any guidance on how to do this?
$\displaystyle \delta_{ij} \delta_{ij} = \sum_{i,j = 1}^n \delta_{ij} \delta_{ij}$
by the Einstein convention.
Now, you know that $\displaystyle \delta_{ij} = 1$ only if i = j, so what is the sum of this series? (ie. how many times in the above sum do we get i = j?)