The LeviCivita symbol, also called the permutation symbol, antisymmetric symbol, or alternating symbol, is a mathematical symbol used in particular in tensor calculus. It is named after the Italian mathematician and physicist Tullio LeviCivita.
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In three dimensions, the LeviCivita symbol is defined as follows:
i.e. is 1 if (i, j, k) is an even permutation of (1,2,3), −1 if it is an odd permutation, and 0 if any index is repeated.
The formula for the three dimensional LeviCivita symbol is:
The formula in four dimensions is:
For example, in linear algebra, the determinant of a 3×3 matrix A can be written
(and similarly for a square matrix of general size, see below)
and the cross product of two vectors can be written as a determinant:
or more simply:
According to the Einstein notation, the summation symbols may be omitted.
The tensor whose components in an orthonormal basis are given by the LeviCivita symbol (a tensor of covariant rank n) is sometimes called the permutation tensor. It is actually a pseudotensor because under an orthogonal transformation of jacobian determinant −1 (i.e., a rotation composed with a reflection), it acquires a minus sign. Because the LeviCivita symbol is a pseudotensor, the result of taking a cross product is a pseudovector, not a vector.
Note that under a general coordinate change, the components of the permutation tensor get multiplied by the jacobian of the transformation matrix. This implies that in coordinate frames different from the one in which the tensor was defined, its components can differ from those of the LeviCivita symbol by an overall factor. If the frame is orthonormal, the factor will be ±1 depending on whether the orientation of the frame is the same or not.
The LeviCivita symbol is related to the Kronecker delta. In three dimensions, the relationship is given by the following equations:
(In Einstein notation, the duplication of the i index implies the sum on i. The previous is then noted: )
The LeviCivita symbol can be generalized to higher dimensions:
Thus, it is the sign of the permutation in the case of a permutation, and zero otherwise.
The generalized formula is:
where n is the dimension (rank).
For any n the property
follows from the facts that (a) every permutation is either even or odd, (b) (+1)^{2} = (1)^{2} = 1, and (c) the permutations of any nelement set number exactly n!.
In indexfree tensor notation, the LeviCivita symbol is replaced by the concept of the Hodge dual.
In general n dimensions one can write the product of two LeviCivita symbols as:
(in these examples, superscripts should be considered equivalent with subscripts)
1. In two dimensions, when all i,j,m,n are in {1,2},
2. In three dimensions, when all i,j,k,m,n are in {1,2,3},
3. In n dimensions, when all i_{1},...,i_{n},j_{1},...,j_{n} are in {1,...,n},:
For equation 1, both sides are antisymmetric with respect of ij and mn. We therefore only need to consider the case and . By substitution, we see that the equation holds for , i.e., for i = m = 1 and j = n = 2. (Both sides are then one). Since the equation is antisymmetric in ij and mn, any set of values for these can be reduced to the above case (which holds). The equation thus holds for all values of ij and mn. Using equation 1, we have for equation 2 =
Here we used the Einstein summation convention with i going from 1 to 2. Equation 3 follows similarly from equation 2. To establish equation 4, let us first observe that both sides vanish when . Indeed, if , then one can not choose m and n such that both permutation symbols on the left are nonzero. Then, with i = j fixed, there are only two ways to choose m and n from the remaining two indices. For any such indices, we have (no summation), and the result follows. Property (5) follows since 3! = 6 and for any distinct indices i,j,k in {1,2,3}, we have (no summation).
1. The determinant of an matrix A = (a_{ij}) can be written as
where each i_{l} should be summed over
Equivalently, it may be written as
where now each i_{l} and each j_{l} should be summed over .
2. If A = (A^{1},A^{2},A^{3}) and B = (B^{1},B^{2},B^{3}) are vectors in R^{3} (represented in some right hand oriented orthonormal basis), then the ith component of their cross product equals
For instance, the first component of is A^{2}B^{3} − A^{3}B^{2}. From the above expression for the cross product, it is clear that . Further, if C = (C^{1},C^{2},C^{3}) is a vector like A and B, then the triple scalar product equals
From this expression, it can be seen that the triple scalar product is antisymmetric when exchanging any adjacent arguments. For example, .
3. Suppose F = (F^{1},F^{2},F^{3}) is a vector field defined on some open set of R^{3} with Cartesian coordinates x = (x^{1},x^{2},x^{3}). Then the ith component of the curl of F equals
A shorthand notation for antisymmetrization is denoted by a pair of square brackets. For example, in arbitrary dimensions, for a rank 2 covariant tensor M,
and for a rank 3 covariant tensor T,
In three dimensions, these are equivalent to
While in four dimensions, these are equivalent to
The LeviCivita symbol may be considered to be a tensor density in two different ways. It may be regarded as a contravariant tensor density of weight +1 or as a covariant tensor density of weight 1. In four dimensions,
This article incorporates material from LeviCivita permutation symbol on PlanetMath, which is licensed under the Creative Commons Attribution/ShareAlike License.
