Antisymmetric tensor

In mathematics and theoretical physics, a tensor is antisymmetric on (or with respect to) an index subset if it alternates sign (+/−) when any two indices of the subset are interchanged. The index subset must generally either be all covariant or all contravariant.

For example,

$T_{ijk\dots} = -T_{jik\dots} = T_{jki\dots} = -T_{kji\dots} = T_{kij\dots} = -T_{ikj\dots}$

holds when the tensor is antisymmetric on it first three indices.

If a tensor changes sign under exchange of any pair of its indices, then the tensor is completely (or totally) antisymmetric. A completely antisymmetric covariant tensor of order p may be referred to as a p-form, and a completely antisymmetric contravariant tensor may be referred to as a p-vector.

Antisymmetric and symmetric tensors

A tensor A that is antisymmetric on indices i and j has the property that the contraction with a tensor B that is symmetric on indices i and j is identically 0.

For a general tensor U with components $U_{ijk\dots}$ and a pair of indices i and j, U has symmetric and antisymmetric parts defined as:

$U_{(ij)k\dots}=\frac{1}{2}(U_{ijk\dots}+U_{jik\dots})$		(symmetric part)
$U_{[ij]k\dots}=\frac{1}{2}(U_{ijk\dots}-U_{jik\dots})$		(antisymmetric part).

Similar definitions can be given for other pairs of indices. As the term "part" suggests, a tensor is the sum of its symmetric part and antisymmetric part for a given pair of indices, as in

$U_{ijk\dots}=U_{(ij)k\dots}+U_{[ij]k\dots}.$

Notation

A shorthand notation for anti-symmetrization is denoted by a pair of square brackets. For example, in arbitrary dimensions, for an order 2 covariant tensor M,

$M_{[ab]} = \frac{1}{2!}(M_{ab} - M_{ba}) ,$

and for an order 3 covariant tensor T,

$T_{[abc]} = \frac{1}{3!}(T_{abc}-T_{acb}+T_{bca}-T_{bac}+T_{cab}-T_{cba}) .$

In any number of dimensions, these are equivalent to

$M_{[ab]} = \frac{1}{2!} \, \delta_{ab}^{cd} M_{cd} ,$

$T_{[abc]} = \frac{1}{3!} \, \delta_{abc}^{def} T_{def} .$

More generally, irrespective of the number of dimensions, antisymmetrization over p indices may be expressed as

$S_{[a_1 \dots a_p]} = \frac{1}{p!} \delta_{a_1 \dots a_p}^{b_1 \dots b_p} S_{b_1 \dots b_p} .$

In the above,

$\delta_{ab\dots}^{cd\dots}$

is the generalized Kronecker delta of the appropriate order.

Examples

Antisymmetric tensors include:

• The electromagnetic tensor, $F_{\mu\nu}$ in electromagnetism
• The Riemannian volume form on a pseudo-Riemannian manifold.

See also

• Levi-Civita symbol
• Symmetric tensor
• Antisymmetric matrix
• Exterior algebra
• Ricci calculus

External links

• [1] - mathworld, wolfram