# Regarding Einstein Summation Convention

1. Sep 30, 2012

### clayton26

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:

εijkajaj = ?

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

2. Oct 1, 2012

### dextercioby

It's nothing to approach, it's incorrect. In a monomial (product of indexed objects) an index can appear no more than 2 times.

3. Oct 1, 2012

### mathman

It looks like a simple error. ajaj should be aiaj