# Permutation Tensor (Explanation Needed)

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• Sep 13th 2011, 05:30 PM
divinelogos
Permutation Tensor (Explanation Needed)
Can somebody help me understand the Permutation Tensor?

I know what it means when it says "if i=j or j=k or i=k, then the permutation tensor is 0".

However, I don't understand the concepts of even and odd permutation (ie, those cases when the permutation tensor equals -1 or 1). Can somebody explain?

Thanks!!! (Rock)
• Sep 14th 2011, 06:37 AM
Ackbeet
Re: Permutation Tensor (Explanation Needed)
A permutation is when you've swapped two indices. So, if I start with the ordering (1,2,3), and I perform one swap (a swap has to be on adjacent numbers), I could get, for example, (2,1,3). Since I've performed one swap, the permutation (2,1,3) is an odd permutation. Now, let's suppose I do another swap from (2,1,3) to get to (2,3,1). I've now done two swaps, so (2,3,1) is an even permutation. The even-ness or odd-ness of a permutation is identified with the even-ness or odd-ness of the number of swaps I must do to get to that permutation. It's a theorem of abstract algebra that, although there are non-unique ways to get to a particular permutation (for example, I might do two swaps and get right back to where I started), the even-ness or odd-ness of a permutation is constant.

Does that help? Incidentally, in practice, the most important thing to know about the Levi-Civita symbol $\varepsilon_{ijk}$ is that

$\sum_{i}\varepsilon_{ijk}\varepsilon_{imn}=\delta_ {jm}\delta_{kn}-\delta_{jn}\delta_{km}.$

And you should know the relationship between the vector cross product and the Levi-Civita symbol as well.