# Linear complex structure

In mathematics, a **complex structure** on a real vector space *V* is an automorphism of *V* that squares to the minus identity, *−I*. Such a structure on *V* allows one to define multiplication by complex scalars in a canonical fashion so as to regard *V* as a complex vector space.

Every complex vector space can be equipped with a compatible complex structure, however, there is in general no canonical such structure. Complex structures have applications in representation theory as well as in complex geometry where they play an essential role in the definition of almost complex manifolds, by contrast to complex manifolds. The term "complex structure" often refers to this structure on manifolds; when it refers instead to a structure on vector spaces, it may be called a "*linear* complex structure".

## Definition and properties

A **complex structure** on a real vector space *V* is a real linear transformation

such that

Here *J*^{2} means *J* composed with itself and Id_{V} is the identity map on *V*. That is, the effect of applying *J* twice is the same as multiplication by −1. This is reminiscent of multiplication by the imaginary unit, *i*. A complex structure allows one to endow *V* with the structure of a complex vector space. Complex scalar multiplication can be defined by

for all real numbers *x*,*y* and all vectors *v* in *V*. One can check that this does, in fact, give *V* the structure of a complex vector space which we denote *V*_{J}.

Going in the other direction, if one starts with a complex vector space *W* then one can define a complex structure on the underlying real space by defining *Jw* = *iw* for all *w* ∈ *W*.

More formally, a linear complex structure on a real vector space is an algebra representation of the complex numbers **C**, thought of as an associative algebra over the real numbers. This algebra is realized concretely as

which corresponds to i^{2} = −1. Then a representation of **C** is a real vector space *V*, together with an action of **C** on *V* (a map **C** → End(*V*)). Concretely, this is just an action of *i*, as this generates the algebra, and the operator representing *i* (the image of *i* in End(*V*)) is exactly *J*.

If *V*_{J} has complex dimension *n* then *V* must have real dimension 2*n*. That is, a finite-dimensional space *V* admits a complex structure only if it is even-dimensional. It is not hard to see that every even-dimensional vector space admits a complex structure. One can define *J* on pairs *e*,*f* of basis vectors by *Je* = *f* and *Jf* = −*e* and then extend by linearity to all of *V*. If (*v*_{1}, …, *v*_{n}) is a basis for the complex vector space *V*_{J} then (*v*_{1}, *Jv*_{1}, …, *v*_{n}, *Jv*_{n}) is a basis for the underlying real space *V*.

A real linear transformation *A* : *V* → *V* is a *complex* linear transformation of the corresponding complex space *V*_{J} if and only if *A* commutes with *J*, i.e. if and only if

Likewise, a real subspace *U* of *V* is a complex subspace of *V*_{J} if and only if *J* preserves *U*, i.e. if and only if

## Examples

### C^{n}

The fundamental example of a linear complex structure is the structure on **R**^{2n} coming from the complex structure on **C**^{n}. That is, the complex *n*-dimensional space **C**^{n} is also a real 2*n*-dimensional space – using the same vector addition and real scalar multiplication – while multiplication by the complex number *i* is not only a *complex* linear transform of the space, thought of as a complex vector space, but also a *real* linear transform of the space, thought of as a real vector space. Concretely, this is because scalar multiplication by *i* commutes with scalar multiplication by real numbers – and distributes across vector addition. As a complex *n*×*n* matrix, this is simply the scalar matrix with *i* on the diagonal. The corresponding real 2*n*×2*n* matrix is denoted *J*.

Given a basis for the complex space, this set, together with these vectors multiplied by *i,* namely form a basis for the real space. There are two natural ways to order this basis, corresponding abstractly to whether one writes the tensor product as or instead as

If one orders the basis as then the matrix for *J* takes the block diagonal form (subscripts added to indicate dimension):

This ordering has the advantage that it respects direct sums of complex vector spaces, meaning here that the basis for is the same as that for

On the other hand, if one orders the basis as then the matrix for *J* is block-antidiagonal:

This ordering is more natural if one thinks of the real space as a direct sum of real spaces, as discussed below.

The data of the real vector space and the *J* matrix is exactly the same as the data of the complex vector space, as the *J* matrix allows one to define complex multiplication. At the level of Lie algebras and Lie groups, this corresponds to the inclusion of gl(*n*,**C**) in gl(2*n*,**R**) (Lie algebras – matrices, not necessarily invertible) and GL(*n*,**C**) in GL(2*n*,**R**):

- gl(
*n*,**C**) < gl(*2n*,**R**) and GL(*n*,**C**) < GL(*2n*,**R**).

The inclusion corresponds to forgetting the complex structure (and keeping only the real), while the subgroup GL(*n*,**C**) can be characterized (given in equations) as the matrices that *commute* with *J:*

The corresponding statement about Lie algebras is that the subalgebra gl(*n*,**C**) of complex matrices are those whose Lie bracket with *J* vanishes, meaning in other words, as the kernel of the map of bracketing with *J,*

Note that the defining equations for these statements are the same, as AJ = JA is the same as which is the same as though the meaning of the Lie bracket vanishing is less immediate geometrically than the meaning of commuting.

### Direct sum

If *V* is any real vector space there is a canonical complex structure on the direct sum *V* ⊕ *V* given by

The block matrix form of *J* is

where is the identity map on *V*. This corresponds to the complex structure on the tensor product

## Compatibility with other structures

If *B* is a bilinear form on *V* then we say that *J* **preserves** *B* if

for all *u*, *v* ∈ *V*. An equivalent characterization is that *J* is skew-adjoint with respect to *B*:

If *g* is an inner product on *V* then *J* preserves *g* if and only if *J* is an orthogonal transformation. Likewise, *J* preserves a nondegenerate, skew-symmetric form ω if and only if *J* is a symplectic transformation (that is, if ω(*Ju*, *Jv*) = ω(*u*, *v*)). For symplectic forms ω there is usually an added restriction for compatibility between *J* and ω, namely

for all *u* in *V*. If this condition is satisfied then *J* is said to **tame** ω.

Given a symplectic form ω and a linear complex structure *J*, one may define an associated symmetric bilinear form *g*_{J} on *V*_{J}

Because a symplectic form is nondegenerate, so is the associated bilinear form. Moreover, the associated form is preserved by *J* if and only if the symplectic form is, and if ω is tamed by *J* then the associated form is positive definite. Thus in this case the associated form is a Hermitian form and *V*_{J} is an inner product space.

## Relation to complexifications

Given any real vector space *V* we may define its complexification by extension of scalars:

This is a complex vector space whose complex dimension is equal to the real dimension of *V*. It has a canonical complex conjugation defined by

If *J* is a complex structure on *V*, we may extend *J* by linearity to *V*^{C}:

Since **C** is algebraically closed, *J* is guaranteed to have eigenvalues which satisfy λ^{2} = −1, namely λ = ±*i*. Thus we may write

where *V*^{+} and *V*^{−} are the eigenspaces of +*i* and −*i*, respectively. Complex conjugation interchanges *V*^{+} and *V*^{−}. The projection maps onto the *V*^{±} eigenspaces are given by

So that

There is a natural complex linear isomorphism between *V*_{J} and *V*^{+}, so these vector spaces can be considered the same, while *V*^{−} may be regarded as the complex conjugate of *V*_{J}.

Note that if *V*_{J} has complex dimension *n* then both *V*^{+} and *V*^{−} have complex dimension *n* while *V*^{C} has complex dimension 2*n*.

Abstractly, if one starts with a complex vector space *W* and takes the complexification of the underlying real space, one obtains a space isomorphic to the direct sum of *W* and its conjugate:

Let *V* be a real vector space with a complex structure *J*. The dual space *V** has a natural complex structure *J** given by the dual (or transpose) of *J*. The complexification of the dual space (*V**)^{C} therefore has a natural decomposition

into the ±*i* eigenspaces of *J**. Under the natural identification of (*V**)^{C} with (*V*^{C})* one can characterize (*V**)^{+} as those complex linear functionals which vanish on *V*^{−}. Likewise (*V**)^{−} consists of those complex linear functionals which vanish on *V*^{+}.

The (complex) tensor, symmetric, and exterior algebras over *V*^{C} also admit decompositions. The exterior algebra is perhaps the most important application of this decomposition. In general, if a vector space *U* admits a decomposition *U* = *S* ⊕ *T* then the exterior powers of *U* can be decomposed as follows:

A complex structure *J* on *V* therefore induces a decomposition

where

All exterior powers are taken over the complex numbers. So if *V*_{J} has complex dimension *n* (real dimension 2*n*) then

The dimensions add up correctly as a consequence of Vandermonde's identity.

The space of (*p*,*q*)-forms Λ^{p,q} *V*_{J}* is the space of (complex) multilinear forms on *V*^{C} which vanish on homogeneous elements unless *p* are from *V*^{+} and *q* are from *V*^{−}. It is also possible to regard Λ^{p,q} *V*_{J}* as the space of real multilinear maps from *V*_{J} to **C** which are complex linear in *p* terms and conjugate-linear in *q* terms.

See complex differential form and almost complex manifold for applications of these ideas.

## See also

- Almost complex manifold
- Complex manifold
- Complex differential form
- Complex conjugate vector space
- Hermitian structure
- Real structure

## References

- Kobayashi S. and Nomizu K., Foundations of Differential Geometry, John Wiley & Sons, 1969. ISBN 0-470-49648-7. (complex structures are discussed in Volume II, Chapter IX, section 1).
- Budinich, P. and Trautman, A.
*The Spinorial Chessboard*, Spinger-Verlag, 1988. ISBN 0-387-19078-3. (complex structures are discussed in section 3.1). - Goldberg S.I.,
*Curvature and Homology*, Dover edition, 1982. ISBN 0-486-64314-X. (complex structures and almost complex manifolds are discussed in section 5.2).