I'm not sure if this is the correct place to ask this question, so please let me know if there is a better place for me to post it. I'm having trouble understanding index notation. I understand the basics, such as in the following examples: (a x b) = εijkajbk εijkεiab = δjaδkbδjbδka δijaj = ai 1. The problem statement, all variables and given/known data Here's the problem I'm trying to solve: Which of the following are allowed in index notation: a = bicijdj a = bici + dj ai = δijbi + ci ak = bicki There are a whole bunch more of these, but I think I can probably figure those out if I get some help with these. Please explain which of those are allowed in index notation and why they are allowed or not allowed. Thanks! 2. Relevant equations 3. The attempt at a solution I'd say that the first is allowed because neither i nor j is repeated more than twice. Same with the second. For the third, I know δijbi = bj but then I'm not sure what to make of it from there. The fourth seems okay since k is the free variable and we're summing over i. Thanks again for your explanations!