Solution
= Solution
\b[This is the incorrectly formed expression.] In the <Einstein summation convention>, an index in a term is either free and occurs once, or summed and occurs twice. Here $i$ occurs three times in the same product, so it cannot consistently be classified as either. Renaming a dummy index before multiplying would give a valid different expression, such as $\delta_{kk}\delta_{ij}=3\delta_{ij}$, but does not make the printed expression valid.