# Is this a type-o or intentional?

1. Sep 6, 2013

### tolove

I've been staring at this confused for a while now, and I've just realized that this might be a type-o. Should I assume that the Levi-Civita symbol is only defined on j,k, and that the i is a type-o, or is there an unwritten rule with this notation that gives the i a meaning?

$\sum$j$\sum$k εi,j,kj,k

2. Sep 6, 2013

### D H

Staff Emeritus
Nominally that kind of sum would represent ith component of a vector. However, since δjk is zero if j≠k and εijk is zero if j=k, this is a complicated way of writing the zero vector.

3. Sep 6, 2013

### Staff: Mentor

Minor pedantic point: The word is "typo", which is short for typographical error.

4. Sep 6, 2013

### tolove

Thank you very much! The i isn't being defined because it doesn't matter what i is in this situation.

And to mark... I didn't know that, thanks

5. Sep 6, 2013

### Ray Vickson

No typo: the standard ε-symbol, used, eg., in writing vector cross-products in 3 dimensions, is:
$$\epsilon_{i j k} = \left\{ \begin{array}{rccl} 1 &\text{ if }& ijk &\text{ is an even permutation of 123}\\ -1&\text{ if }&ijk & \text{ is an odd permutation of 123}\\ 0 &&&\text{ otherwise } \end{array} \right.$$

So, for example, the ith component of $\vec{C} = \vec{A} \times \vec{B}$ is $C_i = \sum_{j,k} \epsilon_{ijk} A_j B_k$.

BTW: 'type-o' is a category of blood (for blood donations); what you probably mean is 'typo'.