Semilinear map
In
- additive with respect to vector addition:
- there exists a field automorphism θ of K such that . If such an automorphism exists and T is nonzero, it is unique, and T is called θ-semilinear.
Where the domain and codomain are the same space (i.e. T : V → V), it may be termed a semilinear transformation. The invertible semilinear transforms of a given vector space V (for all choices of field automorphism) form a group, called the general semilinear group and denoted by analogy with and extending the general linear group. The special case where the field is the complex numbers and the automorphism is complex conjugation, a semilinear map is called an antilinear map.
Similar notation (replacing Latin characters with Greek) are used for semilinear analogs of more restricted linear transform; formally, the
Definition
A map f : V → W for vector spaces V and W over fields K and L respectively is σ-semilinear, or simply semilinear, if there exists a field homomorphism σ : K → L such that for all x, y in V and λ in K it holds that
A given embedding σ of a field K in L allows us to identify K with a subfield of L, making a σ-semilinear map a K-linear map under this identification. However, a map that is τ-semilinear for a distinct embedding τ ≠ σ will not be K-linear with respect to the original identification σ, unless f is identically zero.
More generally, a map ψ : M → N between a right R-module M and a left S-module N is σ-semilinear if there exists a ring antihomomorphism σ : R → S such that for all x, y in M and λ in R it holds that
The term semilinear applies for any combination of left and right modules with suitable adjustment of the above expressions, with σ being a homomorphism as needed.[1][2]
The pair (ψ, σ) is referred to as a dimorphism.[3]
Related
Transpose
Let be a ring isomorphism, a right -module and a right -module, and a -semilinear map. Define the transpose of as the mapping that satisfies[4]
Properties
Let be a ring isomorphism, a right -module and a right -module, and a -semilinear map. The mapping
Examples
- Let with standard basis . Define the map by
- f is semilinear (with respect to the complex conjugation field automorphism) but not linear.
- Let – the Galois field of order , p the characteristic. Let . By the Freshman's dream it is known that this is a field automorphism. To every linear map between vector spaces V and W over K we can establish a -semilinear map
- Indeed every linear map can be converted into a semilinear map in such a way. This is part of a general observation collected into the following result.
- Let be a noncommutative ring, a left -module, and an invertible element of . Define the map , so , and is an inner automorphism of . Thus, the homothetyneed not be a linear map, but is -semilinear.[6]
General semilinear group
Given a vector space V, the set of all invertible semilinear transformations V → V (over all field automorphisms) is the group ΓL(V).
Given a vector space V over K, ΓL(V) decomposes as the semidirect product
where Aut(K) is the automorphisms of K. Similarly, semilinear transforms of other linear groups can be defined as the semidirect product with the automorphism group, or more intrinsically as the group of semilinear maps of a vector space preserving some properties.
We identify Aut(K) with a subgroup of ΓL(V) by fixing a basis B for V and defining the semilinear maps:
for any . We shall denoted this subgroup by Aut(K)B. We also see these complements to GL(V) in ΓL(V) are acted on regularly by GL(V) as they correspond to a change of basis.
Proof
Every linear map is semilinear, thus . Fix a basis B of V. Now given any semilinear map f with respect to a field automorphism σ ∈ Aut(K), then define g : V → V by
As f(B) is also a basis of V, it follows that g is simply a basis exchange of V and so linear and invertible: g ∈ GL(V).
Set . For every in V,
thus h is in the Aut(K) subgroup relative to the fixed basis B. This factorization is unique to the fixed basis B. Furthermore, GL(V) is normalized by the action of Aut(K)B, so ΓL(V) = GL(V) ⋊ Aut(K).
Applications
Projective geometry
The groups extend the typical classical groups in GL(V). The importance in considering such maps follows from the consideration of projective geometry. The induced action of on the associated projective space P(V) yields the projective semilinear group, denoted , extending the projective linear group, PGL(V).
The projective geometry of a vector space V, denoted PG(V), is the lattice of all subspaces of V. Although the typical semilinear map is not a linear map, it does follow that every semilinear map induces an order-preserving map . That is, every semilinear map induces a
Mathieu group
The group PΓL(3,4) can be used to construct the Mathieu group M24, which is one of the
See also
- Antilinear map
- Complex conjugate vector space
References
- ^ Ian R. Porteous (1995), Clifford Algebras and the Classical Groups, Cambridge University Press
- ^ Bourbaki (1989), Algebra I (2nd ed.), Springer-Verlag, p. 223
- ^ Bourbaki (1989), Algebra I (2nd ed.), Springer-Verlag, p. 223
- ^ Bourbaki (1989), Algebra I (2nd ed.), Springer-Verlag, p. 236
- ^ Bourbaki (1989), Algebra I (2nd ed.), Springer-Verlag, p. 236
- ^ Bourbaki (1989), Algebra I (2nd ed.), Springer-Verlag, p. 223
- Assmus, E.F.; Key, J.D. (1994), Designs and Their Codes, ISBN 0-521-45839-0
- OCLC 18588156.
- Bray, John N.; Holt, Derek F.; MR 2502211
- Faure, Claude-Alain; Frölicher, Alfred (2000), Modern Projective Geometry, ISBN 0-7923-6525-9
- Gruenberg, K.W.; Weir, A.J. (1977), Linear Geometry, Graduate Texts in Mathematics, vol. 49 (1st ed.), Springer-Verlag New York
- OCLC 853623322.
This article incorporates material from semilinear transformation on