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## Main Question or Discussion Point

So, I realize the basic theory behind Einstein Summation Convention is that any repeated set of indices implicitly indicates a sum over those indices. However, what if an index is repeated three times?

For example, my mathematics professor posted this problem:

ε

As you can see, j is repeated thrice. So how do I approach this?

For example, my mathematics professor posted this problem:

ε

_{ijk}a_{j}a_{j}= ?As you can see, j is repeated thrice. So how do I approach this?