Levi-Civita symbol

Not to be confused with Levi-Civita connection.

In mathematics, particularly in linear algebra, tensor analysis, and differential geometry, the Levi-Civita symbol represents a collection of numbers; defined from the sign of a permutation of the natural numbers 1, 2, …, n, for some positive integer n. It is named after the Italian mathematician and physicist Tullio Levi-Civita. Other names include the permutation symbol, antisymmetric symbol, or alternating symbol, which refer to its antisymmetric property and definition in terms of permutations.

The standard letters to denote the Levi-Civita symbol are the Greek lower case epsilon ε or ϵ, or less commonly the Latin lower case e. Index notation allows one to display permutations in a way compatible with tensor analysis:

where each index i1, i2, …, in takes values 1, 2, …, n. There are nn indexed values of εi1i2in, which can be arranged into an n-dimensional array. The key definitive property of the symbol is total antisymmetry in all the indices. When any two indices are interchanged, equal or not, the symbol is negated:

If any two indices are equal, the symbol is zero. When all indices are unequal, we have:

where p (called the parity of the permutation) is the number of interchanges of indices necessary to unscramble i1, i2, …, in into the order 1, 2, …, n, and the factor (−1)p is called the sign or signature of the permutation. The value ε12…n must be defined, else the particular values of the symbol for all permutations are indeterminate. Most authors choose ε12…n = +1, which means the Levi-Civita symbol equals the sign of a permutation when the indices are all unequal. This choice is used throughout this article.

The term "n-dimensional Levi-Civita symbol" refers to the fact that the number of indices on the symbol n matches the dimensionality of the relevant vector space in question, which may be Euclidean or non-Euclidean, pure space or spacetime. The values of the Levi-Civita symbol are independent of any metric tensor and coordinate system. Also, the specific term "symbol" emphasizes that it is not a tensor because of how it transforms between coordinate systems, however it can be interpreted as a tensor density.

The Levi-Civita symbol allows the determinant of a square matrix, and the cross product of two vectors in three-dimensional Euclidean space, to be expressed in index notation.

Definition

The common dimensionalities of the Levi-Civita symbol are in three and four dimensions, and to some extent two dimensions, so it is useful to see these definitions before the general one in any number of dimensions.

Two dimensions

The two-dimensional Levi-Civita symbol is defined by:

The values can be arranged into a 2 × 2 antisymmetric matrix:

Use of the two-dimensional symbol is relatively uncommon, although in certain specialized topics like supersymmetry[1] and twistor theory[2] it appears in the context of 2-spinors. The three- and higher-dimensional Levi-Civita symbols are used more commonly.

Three dimensions

For the indices (i, j, k) in εijk, the values 1, 2, 3 occurring in the      cyclic order (1, 2, 3) correspond to ε = +1, while occurring in the      reverse cyclic order correspond to ε = −1, otherwise ε = 0.

In three dimensions, the Levi-Civita symbol is defined as follows:[3]

That is, εijk 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. In three dimensions only, the cyclic permutations of (1, 2, 3) are all even permutations, similarly the anticyclic permutations are all odd permutations. This means in 3d it is sufficient to take cyclic or anticyclic permutations of (1,2,3) and easily obtain all the even or odd permutations.

Analogous to 2-dimensional matrices, the values of the 3-dimensional Levi-Civita symbol can be arranged into a 3 × 3 × 3 array:

where i is the depth, j the row and k the column.

Some examples:

Four dimensions

In four dimensions, the Levi-Civita symbol is defined as:

These values can be arranged into a 4 × 4 × 4 × 4 array, although in 4 dimensions and higher this is difficult to draw.

Some examples:

Generalization to n dimensions

The Levi-Civita symbol can be generalized to n dimensions:[4]

Thus, it is the sign of the permutation in the case of a permutation, and zero otherwise.

Using the capital pi notation for ordinary multiplication of numbers, an explicit expression for the symbol is:

where the product is totally antisymmetric in all indices, and the sign function (denoted by "sgn") extracts the sign of each difference discarding the absolute value. The formula is valid for all index values, and for any n (when n = 0 or 1, this is the empty product). However, computing the formula above naively is O(n2) in time complexity, whereas the sign can be computed via the disjoint cycles of a permutation in only O(n log(n)) cost.

Properties

A tensor whose components in an orthonormal basis are given by the Levi-Civita symbol (a tensor of covariant rank n) is sometimes called a 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. As the Levi-Civita symbol is a pseudotensor, the result of taking a cross product is a pseudovector, not a vector.[5]

Under a general coordinate change, the components of the permutation tensor are 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 Levi-Civita 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.[5]

In index-free tensor notation, the Levi-Civita symbol is replaced by the concept of the Hodge dual.

In a context where tensor index notation is used to manipulate tensor components, the Levi-Civita symbol may be written with its indices as either subscripts or superscripts with no change in meaning, as might be convenient. Thus, one could write

In these examples, superscripts should be considered equivalent with subscripts.

Summation symbols can be eliminated by using Einstein notation, where an index repeated between two or more terms indicates summation over that index. For example,

.

In the following examples, Einstein notation is used.

Two dimensions

In two dimensions, when all i, j, m, n each take the values 1 and 2,[3]

( 1 )

( 2 )

( 3 )

Three dimensions

Index and symbol values

In three dimensions, when all i, j, k, m, n each take values 1, 2, and 3:[3]

( 4 )

( 5 )

( 6 )

Product

The Levi-Civita symbol is related to the Kronecker delta. In three dimensions, the relationship is given by the following equations (vertical lines denote the determinant):[4]

A special case of this result is (4):

sometimes called the "contracted epsilon identity".

In Einstein notation, the duplication of the i index implies the sum on i. The previous is then denoted εijkεimn = δjmδknδjnδkm.

n dimensions

Index and symbol values

In n dimensions, when all i1, …,in, j1, …, jn take values 1, 2, …, n:

( 7 )

( 8 )

( 9 )

where the exclamation mark (!) denotes the factorial, and δα
β
is the generalized Kronecker delta. For any n, the property

follows from the facts that

• every permutation is either even or odd,
• (+1)2 = (−1)2 = 1, and
• the number of permutations of any n-element set number is exactly n!.

Product

In general, for n dimensions, one can write the product of two Levi-Civita symbols as:

.

Proofs

For (1), both sides are antisymmetric with respect of ij and mn. We therefore only need to consider the case ij and mn. By substitution, we see that the equation holds for ε12ε12, 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 (1), we have for (2)

Here we used the Einstein summation convention with i going from 1 to 2. Next, (3) follows similarly from (2).

To establish (5), notice that both sides vanish when ij. Indeed, if ij, 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.

Then (6) follows since 3! = 6 and for any distinct indices i, j, k taking values 1, 2, 3, we have

(no summation, distinct i, j, k)

Applications and examples

Determinants

In linear algebra, the determinant of a 3 × 3 square matrix A = [aij] can be written[6]

Similarly the determinant of an n × n matrix A = [aij] can be written as[5]

where each ir should be summed over 1, …, n, or equivalently:

where now each ir and each jr should be summed over 1, …, n. More generally, we have the identity[5]

Vector cross product

Main article: cross product

Cross product (two vectors)

If a = (a1, a2, a3) and b = (b1, b2, b3) are vectors in 3 (represented in some right-handed coordinate system using an orthonormal basis), their cross product can be written as a determinant:[5]

hence also using the Levi-Civita symbol, and more simply:

In Einstein notation, the summation symbols may be omitted, and the ith component of their cross product equals[4]

The first component is

then by cyclic permutations of 1, 2, 3 the others can be derived immediately, without explicitly calculating them from the above formulae:

Triple scalar product (three vectors)

From the above expression for the cross product, we have:

.

If c = (c1, c2, c3) is a third vector, then the triple scalar product equals

From this expression, it can be seen that the triple scalar product is antisymmetric when exchanging any pair of arguments. For example,

.

Curl (one vector field)

If F = (F1, F2, F3) is a vector field defined on some open set of 3 as a function of position x = (x1, x2, x3) (using Cartesian coordinates). Then the ith component of the curl of F equals[4]

which follows from the cross product expression above, substituting components of the gradient vector operator (nabla).

Tensor density

In any arbitrary curvilinear coordinate system and even in the absence of a metric on the manifold, the Levi-Civita symbol as defined above may be considered to be a tensor density field 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 n dimensions using the generalized Kronecker delta,[7][8]

Notice that these are numerically identical. In particular, the sign is the same.

Levi-Civita tensors

On a pseudo-Riemannian manifold, one may define coordinate-invariant covariant and contravariant tensor fields whose coordinate representations agree with the Levi-Civita symbol wherever the coordinate system is such that the basis of the tangent space is orthonormal with respect to the metric and matches a selected orientation. These tensors should not be confused with each other, nor should they be confused with the tensor density fields mentioned above. The covariant Levi-Civita tensor (also known as the Riemannian volume form) in a given coordinate system is

where gab is the representation of the metric in that coordinate system. This tensor may be converted to the contravariant tensor by raising the indices with the metric as usual, but a minus sign is needed if the metric signature contains an odd number of negatives.

where s is the number of negatives in the signature. This results in the following:

Example: Minkowski Space

In Minkowski space (the four-dimensional spacetime of special relativity), the covariant Levi-Civita tensor is

but the contravariant Levi-Civita tensor is

Notice the minus sign. The following are identities.

Notes

1. Labelle, P. (2010). Supersymmetry. Demystified. McGraw-Hill. pp. 57–58. ISBN 978-0-07-163641-4.
2. Hadrovich, F. "Twistor Primer". Retrieved 2013-09-03.
3. Tyldesley, J. R. (1973). An introduction to Tensor Analysis: For Engineers and Applied Scientists. Longman. ISBN 0-582-44355-5.
4. Kay, D. C. (1988). Tensor Calculus. Schaum’s Outlines. McGraw Hill. ISBN 0-07-033484-6.
5. Riley, K. F.; Hobson, M. P.; Bence, S. J. (2010). Mathematical Methods for Physics and Engineering. Cambridge University Press. ISBN 978-0-521-86153-3.
6. Lipcshutz, S.; Lipson, M. (2009). Linear Algebra. Schaum’s Outlines (4th ed.). McGraw Hill. ISBN 978-0-07-154352-1.
7. Murnaghan, F. D. (1925), "The generalized Kronecker symbol and its application to the theory of determinants", Amer. Math. Monthly, 32: 233–241, doi:10.2307/2299191
8. Lovelock, David; Rund, Hanno (1989). Tensors, Differential Forms, and Variational Principles. Courier Dover Publications. p. 113. ISBN 0-486-65840-6.

References

• Wheeler, J. A.; Misner, C.; Thorne, K. S. (1973). Gravitation. W. H. Freeman & Co. pp. 85–86, §3.5. ISBN 0-7167-0344-0.
• Neuenschwander, D. E. (2015). Tensor Calculus for Physics. Johns Hopkins University Press. pp. 11, 29, 95. ISBN 978-1-4214-1565-9.