Complex conjugate of a vector space

In mathematics, the complex conjugate of a complex vector space is a complex vector space that has the same elements and additive group structure as but whose scalar multiplication involves conjugation of the scalars. In other words, the scalar multiplication of satisfies
where is the scalar multiplication of and is the scalar multiplication of
The letter stands for a vector in is a complex number, and denotes the complex conjugate of
More concretely, the complex conjugate vector space is the same underlying vector space with the conjugate linear complex structure .

Motivation

If and are complex vector spaces, a function is antilinear if
With the use of the conjugate vector space, an antilinear map can be regarded as an ordinary linear map of type The linearity is checked by noting:
Conversely, any linear map defined on gives rise to an antilinear map on
This is the same underlying principle as in defining the opposite ring so that a right -module can be regarded as a left -module, or that of an opposite category so that a contravariant functor can be regarded as an ordinary functor of type

Complex conjugation functor

A linear map gives rise to a corresponding linear map that has the same action as Note that preserves scalar multiplication because
Thus, complex conjugation and define a functor from the category of complex vector spaces to itself.
If and are finite-dimensional and the map is described by the complex matrix with respect to the bases of and of then the map is described by the complex conjugate of with respect to the bases of and of

Structure of the conjugate

The vector spaces and have the same dimension over the complex numbers and are therefore isomorphic as complex vector spaces. However, there is no natural isomorphism from to
The double conjugate is identical to

Complex conjugate of a Hilbert space

Given a Hilbert space, its complex conjugate is the same vector space as its continuous dual space
There is one-to-one antilinear correspondence between continuous linear functionals and vectors.
In other words, any continuous linear functional on is an inner multiplication to some fixed vector, and vice versa.
Thus, the complex conjugate to a vector particularly in finite dimension case, may be denoted as .
In quantum mechanics, the conjugate to a ket vector is denoted as – a bra vector.