11/8/13 Complex conjugate - Wikipedia, the free encyclopedia

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Geometric representation of z and its

conjugate z̅ in the complex plane. The

complex conjugate is found by

reflecting z across the real axis.

Complex conjugateFrom Wikipedia, the free encyclopedia

In mathematics, complex conjugates are a pair of complex numbers,both having the same real part, but with imaginary parts of equal

magnitude and opposite signs.[1][2] For example, 3 + 4i and 3 − 4iare complex conjugates.

The conjugate of the complex number

where and are real numbers, is

For example,

An alternative notation for the complex conjugate is . However,

the notation avoids confusion with the notation for the conjugatetranspose of a matrix, which can be thought of as a generalization ofcomplex conjugation. The star-notation is preferred in physics, wheredagger is used for the conjugate transpose, while the bar-notation is more common in pure mathematics. If acomplex number is represented as a 2×2 matrix, the notations are identical.

Complex numbers are considered points in the complex plane, a variation of the Cartesian coordinate systemwhere both axes are real number lines that cross at the origin, however, the y-axis is a product of real numbersmultiplied by . On the illustration, the x-axis is called the real axis, labeled Re, while the y-axis is called theimaginary axis, labeled Im. The plane defined by the Re and Im axes represents the space of all possiblecomplex numbers. In this view, complex conjugation corresponds to reflection of a complex number at the x-axis, equivalent to a 180 degree rotation of the complex plane about the Re axis.

In polar form, the conjugate of is . This can be shown using Euler's formula.

Pairs of complex conjugates are significant because the imaginary unit is qualitatively indistinct from its additiveand multiplicative inverse , as they both satisfy the definition for the imaginary unit: . Thus in most

"natural" settings, if a complex number provides a solution to a problem, so does its conjugate, such as is thecase for complex solutions of the quadratic formula with real coefficients.

In some texts, the complex conjugate of a previous known number is abbreviated as "c.c.". For example, writing means

Contents

1 Properties

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2 Use as a variable3 Generalizations

4 See also

5 Notes

6 References

Properties

These properties apply for all complex numbers z and w, unless stated otherwise, and can be proven by writingz and w in the form a + ib.

if and only if z is real

for any integer n

, involution (i.e., the conjugate of the conjugate of a complex number z is again that number)

if z is non-zero

The latter formula is the method of choice to compute the inverse of a complex number if it is given inrectangular coordinates.

if z is non-zero

In general, if is a holomorphic function whose restriction to the real numbers is real-valued, and is

defined, then

Consequently, if is a polynomial with real coefficients, and , then as well. Thus, non-

real roots of real polynomials occur in complex conjugate pairs (see Complex conjugate root theorem).

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The map from to is a homeomorphism (where the topology on is taken to be the standard

topology) and antilinear, if one considers as a complex vector space over itself. Even though it appears to bea well-behaved function, it is not holomorphic; it reverses orientation whereas holomorphic functions locallypreserve orientation. It is bijective and compatible with the arithmetical operations, and hence is a fieldautomorphism. As it keeps the real numbers fixed, it is an element of the Galois group of the field extension

. This Galois group has only two elements: and the identity on . Thus the only two field

automorphisms of that leave the real numbers fixed are the identity map and complex conjugation.

Use as a variable

Once a complex number or is given, its conjugate is sufficient to reproduce the parts

of the z-variable:

Real part:

Imaginary part:

Modulus/absolute value:

Argument: , so

Thus the pair of variables and also serve up the plane as do x,y and and . Furthermore, the variable isuseful in specifying lines in the plane:

is a line through the origin and perpendicular to since the real part of is zero only when the cosine of theangle between and is zero. Similarly, for a fixed complex unit u = exp(b i), the equation:

determines the line through in the direction of u.

These uses of the conjugate of z as a variable are illustrated in Frank Morley's book Inversive Geometry(1933), written with his son Frank Vigor Morley.

Generalizations

The other planar real algebras, dual numbers, and split-complex numbers are also explicated by use of complexconjugation.

For matrices of complex numbers , where represents the element-by-element

conjugation of .[3] Contrast this to the property , where represents the conjugate

transpose of .

Taking the conjugate transpose (or adjoint) of complex matrices generalizes complex conjugation. Even moregeneral is the concept of adjoint operator for operators on (possibly infinite-dimensional) complex Hilbertspaces. All this is subsumed by the *-operations of C*-algebras.

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One may also define a conjugation for quaternions and coquaternions: the conjugate of is

.

Note that all these generalizations are multiplicative only if the factors are reversed:

Since the multiplication of planar real algebras is commutative, this reversal is not needed there.

There is also an abstract notion of conjugation for vector spaces over the complex numbers. In this context,any antilinear map that satisfies

1. , where and is the identity map on ,

2. for all , , and

3. for all , ,

is called a complex conjugation, or a real structure. As the involution is antilinear, it cannot be the identity

map on . Of course, is a -linear transformation of , if one notes that every complex space V has a real

form obtained by taking the same vectors as in the original space and restricting the scalars to be real. The

above properties actually define a real structure on the complex vector space .[4] One example of this notionis the conjugate transpose operation of complex matrices defined above. It should be remarked that on genericcomplex vector spaces there is no canonical notion of complex conjugation.

See also

Complex conjugate vector space

Real structure

Notes

1. ^ Weisstein, Eric W., "Complex Conjugates (http://mathworld.wolfram.com/ComplexConjugate.html)",MathWorld.

2. ^ Weisstein, Eric W., "Imaginary Numbers (http://mathworld.wolfram.com/ImaginaryNumber.html)",MathWorld.

3. ^ Arfken, Mathematical Methods for Physicists, 1985, pg. 201

4. ^ Budinich, P. and Trautman, A. The Spinorial Chessboard. Spinger-Verlag, 1988, p. 29

References

Budinich, P. and Trautman, A. The Spinorial Chessboard. Spinger-Verlag, 1988. ISBN 0-387-19078-3. (antilinear maps are discussed in section 3.3).

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