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Concept
Real structure
Résumé
In mathematics, a real structure on a complex vector space is a way to decompose the complex vector space in the direct sum of two real vector spaces. The prototype of such a structure is the field of complex numbers itself, considered as a complex vector space over itself and with the conjugation map , with , giving the "canonical" real structure on , that is .
The conjugation map is antilinear: and .
A real structure on a complex vector space V is an antilinear involution . A real structure defines a real subspace , its fixed locus, and the natural map
is an isomorphism. Conversely any vector space that is the complexification
of a real vector space has a natural real structure.
One first notes that every complex space V has a realification obtained by taking the same vectors as in the original set and restricting the scalars to be real. If and then the vectors and are linearly independent in the realification of V. Hence:
Naturally, one would wish to represent V as the direct sum of two real vector spaces, the "real and imaginary parts of V". There is no canonical way of doing this: such a splitting is an additional real structure in V. It may be introduced as follows. Let be an antilinear map such that , that is an antilinear involution of the complex space V.
Any vector can be written ,
where and .
Therefore, one gets a direct sum of vector spaces where:
and .
Both sets and are real vector spaces. The linear map , where , is an isomorphism of real vector spaces, whence:
The first factor is also denoted by and is left invariant by , that is . The second factor is
usually denoted by . The direct sum reads now as:
i.e. as the direct sum of the "real" and "imaginary" parts of V. This construction strongly depends on the choice of an antilinear involution of the complex vector space V. The complexification of the real vector space , i.e.,
admits
a natural real structure and hence is canonically isomorphic to the direct sum of two copies of :
It follows a natural linear isomorphism between complex vector spaces with a given real structure.
Source officielle
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