In mathematics, more specifically in multilinear algebra, an alternating multilinear map is a multilinear map with all arguments belonging to the same vector space (for example, a bilinear form or a multilinear form) that is zero whenever any pair of its arguments is equal. This generalizes directly to a module over a commutative ring.
The notion of alternatization (or alternatisation) is used to derive an alternating multilinear map from any multilinear map of which all arguments belong to the same space.
Definition
Let be a commutative ring and , be modules over . A multilinear map of the form is said to be alternating if it satisfies the following equivalent conditions:
Vector spaces
Let be vector spaces over the same field. Then a multilinear map of the form is alternating if it satisfies the following condition:
- if are linearly dependent then .
Example
In a Lie algebra, the Lie bracket is an alternating bilinear map. The determinant of a matrix is a multilinear alternating map of the rows or columns of the matrix.
Properties
If any component of an alternating multilinear map is replaced by for any and in the base ring , then the value of that map is not changed.[3]
Every alternating multilinear map is antisymmetric,[4] meaning that[1]
or equivalently,
where denotes the permutation group of degree and is the sign of .[5] If is a unit in the base ring , then every antisymmetric -multilinear form is alternating.
Alternatization
Given a multilinear map of the form the alternating multilinear map defined by
is said to be the alternatization of .
Properties
- The alternatization of an -multilinear alternating map is times itself.
- The alternatization of a symmetric map is zero.
- The alternatization of a bilinear map is bilinear. Most notably, the alternatization of any cocycle is bilinear. This fact plays a crucial role in identifying the second cohomology group of a lattice with the group of alternating bilinear forms on a lattice.
See also
Notes
- 1 2 3 Lang 2002, pp. 511–512
- ↑ Bourbaki 2007, A III.80, §4
- 1 2 Dummit & Foote 2004, p. 436
- ↑ Rotman 1995, p. 235
- ↑ Tu 2011, p. 23
References
- Bourbaki, N. (2007). Eléments de mathématique. Vol. Algèbre Chapitres 1 à 3 (reprint ed.). Springer.
- Dummit, David S.; Foote, Richard M. (2004). Abstract Algebra (3rd ed.). Wiley.
- Lang, Serge (2002). Algebra. Graduate Texts in Mathematics. Vol. 211 (revised 3rd ed.). Springer. ISBN 978-0-387-95385-4. OCLC 48176673.
- Rotman, Joseph J. (1995). An Introduction to the Theory of Groups. Graduate Texts in Mathematics. Vol. 148 (4th ed.). Springer. ISBN 0-387-94285-8. OCLC 30028913.
- Tu, Loring W. (2011). An Introduction to Manifolds. Springer-Verlag New York. ISBN 978-1-4419-7400-6.