Chemistry - Levi-Civita symbol

In mathematics, and in particular in tensor calculus, the Levi-Civita symbol, also called the permutation symbol, is defined as follows:

$\epsilon_{ijk} = \left\{ \begin{matrix} +1 & \mbox{if } (i,j,k) \mbox{ is } (1,2,3), (2,3,1) \mbox{ or } (3,1,2)\\ -1 & \mbox{if } (i,j,k) \mbox{ is } (3,2,1), (1,3,2) \mbox{ or } (2,1,3)\\ 0 & \mbox{otherwise: }i=j \mbox{ or } j=k \mbox{ or } k=i \end{matrix} \right.$

It is named after Tullio Levi-Civita. It is used in many areas of mathematics and physics. For example, in linear algebra, the cross product of two vectors can be written as:

$\mathbf{a \times b} = \begin{vmatrix} \mathbf{e_1} & \mathbf{e_2} & \mathbf{e_3} \\ a_1 & a_2 & a_3 \\ b_1 & b_2 & b_3 \\ \end{vmatrix} = \sum_{i,j,k=1}^3 \epsilon_{ijk} \mathbf{e_i} a_j b_k$

or more simply:

$\mathbf{a \times b} = \mathbf{c},\ c_i = \sum_{j,k=1}^3 \epsilon_{ijk} a_j b_k$

This can be further simplified by using Einstein notation.

The Levi-Civita symbol can be generalized to higher dimensions:

$\epsilon_{ijkl\dots} = \left\{ \begin{matrix} +1 & \mbox{if }(i,j,k,l,\dots) \mbox{ is an even permutation of } (1,2,3,4,\dots) \\ -1 & \mbox{if }(i,j,k,l,\dots) \mbox{ is an odd permutation of } (1,2,3,4,\dots) \\ 0 & \mbox{if any two labels are the same} \end{matrix} \right.$

See even permutation or symmetric group for a definition of 'even permutation' and 'odd permutation'

The tensor whose components are given by the Levi-Civita symbol (a tensor of covariant rank n) is sometimes called the permutation tensor. It is actually a pseudotensor because it get a minus sign under orthogonal transformation of jacobian determinant -1 (i.e. a rotation composed with a reflection).

The Levi-Civita symbol is related to the Kronecker delta. In three dimensions, the relationship is given by the following equations:

$\sum_{i=1}^3 \epsilon_{ijk}\epsilon_{imn} = \delta_{jm}\delta_{kn} - \delta_{jn}\delta_{km}$

$\sum_{i,j=1}^3 \epsilon_{ijk}\epsilon_{ijn} = 2\delta_{kn}$

Furthermore, it can be shown that

$\sum_{i,j,k,\dots=1}^n \epsilon_{ijk\dots}\epsilon_{ijk\dots} = n!$

is always fulfilled in n dimensions.

Categories: Linear algebra