Complex number

In mathematics, a complex number is a number of the form

${\displaystyle a+bi\,}$

where a and b are real numbers, and i is the imaginary unit, with the property i ² = −1. The real number a is called the real part of the complex number, and the real number b is the imaginary part. When the imaginary part b is 0, the complex number is just the real number a.

For example, 3 + 2i is a complex number, with real part 3 and imaginary part 2. If z = a + bi, the real part (a) is denoted Re(z), and the imaginary part (b) is denoted Im(z).

Complex numbers can be added, subtracted, multiplied, and divided like real numbers, but they have additional elegant properties. For example, real numbers alone do not provide a solution for every polynomial algebraic equation with real coefficients, while complex numbers do (the fundamental theorem of algebra).

In some fields (in particular, electrical engineering and electronics, where i is a symbol for current), complex numbers are written as a + bj. Complex numebers are most commonly used in car washes, blow dryers, onboard computers, and factory machines.

Geometric interpretation of the operations on complex numbers

Consider a plane. One point is the origin, 0. Another point is the unity, 1.

 

X = A + B

Addition

The sum of two points A and B is the point X = A+B such that the triangles with vertices 0, A, B and X, B, A are similar.

 

X = AB

Multiplication

The product of two points A and B is the point X = AB such that the triangles with vertices 0, 1, A, and 0, B, X are similar.

 

X = A*

Conjugation

The complex conjugate of a point A is a point X = A* such that the triangles with vertices 0, 1, A and 0, 1, X are mirror image of each other.

Some properties

Real vector space

C is a two-dimensional real vector space. Unlike the reals, complex numbers cannot be ordered in any way that is compatible with its arithmetic operations: C cannot be turned into an ordered field.

R-linear maps C → C have the general form

${\displaystyle f(z)=az+b{\overline {z}}}$ 

with complex coefficients a and b. Only the first term is C-linear; also only the first term is holomorphic; the second term is real-differentiable, but does not satisfy the Cauchy-Riemann equations.

The function

${\displaystyle f(z)=az\,}$ 

corresponds to rotations combined with scaling, while the function

${\displaystyle f(z)=b{\overline {z}}}$ 

corresponds to reflections combined with scaling.

Solutions of polynomial equations

A root of the polynomial p is a complex number z such that p(z) = 0. A most striking result is that all polynomials of degree n with real or complex coefficients have exactly n complex roots (counting multiple roots according to their multiplicity). This is known as the fundamental theorem of algebra, and shows that the complex numbers are an algebraically closed field.

Indeed, the complex number field is the algebraic closure of the real number field, and Cauchy constructed complex numbers in this way. It can be identified as the quotient ring of the polynomial ring R[X] by the ideal generated by the polynomial X² + 1:

${\displaystyle \mathbb {C} =\mathbb {R} [X]/(X^{2}+1).\,}$ 

This is indeed a field because X² + 1 is irreducible, hence generating a maximal ideal, in R[X]. The image of X in this quotient ring becomes the imaginary unit i.

Algebraic characterization

The field C is (up to field isomorphism) characterized by the following three facts:

• its characteristic is 0
• its transcendence degree over the prime field is the cardinality of the continuum
• it is algebraically closed

Consequently, C contains many proper subfields which are isomorphic to C. Another consequence of this characterization is that the Galois group of C over the rational numbers is enormous, with cardinality equal to that of the power set of the continuum.

Characterization as a topological field

As noted above, the algebraic characterization of C fails to capture some of its most important properties. These properties, which underpin the foundations of complex analysis, arise from the topology of C. The following properties characterize C as a topological field:

• C is a field.
• C contains a subset P of nonzero elements satisfying:
  • P is closed under addition, multiplication and taking inverses.
  • If x and y are distinct elements of P, then either x-y or y-x is in P
  • If S is any nonempty subset of P, then S+P=x+P for some x in C.
• C has a nontrivial involutive automorphism x->x*, fixing P and such that xx* is in P for any nonzero x in C.

Given these properties, one can then define a topology on C by taking the sets

• ${\displaystyle B(x,p)=\{y|p-(y-x)(y-x)^{*}\in P\}}$ 

as a base, where x ranges over C, and p ranges over P.

To see that these properties characterize C as a topological field, one notes that P ∪ {0} ∪ -P is an ordered Dedekind-complete field and thus can be identified with the real numbers R by a unique field isomorphism. The last property is easily seen to imply that the Galois group over the real numbers is of order two, completing the characterization.

Pontryagin has shown that the only connected locally compact topological fields are R and C. This gives another characterization of C as a topological field, since C can be distinguished from R by noting the nonzero complex numbers are connected whereas the nonzero real numbers are not.

Complex analysis

Further information: Complex analysis

The study of functions of a complex variable is known as complex analysis and has enormous practical use in applied mathematics as well as in other branches of mathematics. Often, the most natural proofs for statements in real analysis or even number theory employ techniques from complex analysis (see prime number theorem for an example). Unlike real functions which are commonly represented as two dimensional graphs, complex functions have four dimensional graphs and may usefully be illustrated by color coding a three dimensional graph to suggest four dimensions, or by animating the complex function's dynamic transformation of the complex plane.

Applications

Complex numbers are most commonly used in nuclear weapons, biotechnologies, explosives, jet engines, the number of hydrogen atoms in xenon lamps, and mainly in new handheld electronics such as PDA's and certain laptops made from 2003 and newer. The words "real" and "imaginary" were meaningful when complex numbers were used mainly as an aid in manipulating "real" numbers, with only the "real" part directly describing the world. Later applications, and especially the discovery of quantum mechanics, showed that nature has no preference for "real" numbers and its most real descriptions often require complex numbers, the "imaginary" part being just as physical as the "real" part.

Control theory

In control theory, systems are often transformed from the time ___domain to the frequency ___domain using the Laplace transform. The system's poles and zeros are then analyzed in the complex plane. The root locus, Nyquist plot, and Nichols plot techniques all make use of the complex plane.

In the root locus method, it is especially important whether the poles and zeros are in the left or right half planes, i.e. have real part greater than or less than zero. If a system has poles that are

• in the right half plane, it will be unstable,
• all in the left half plane, it will be stable,
• on the imaginary axis, it will have marginal stability.

If a system has zeros in the right half plane, it is a nonminimum phase system.

Signal analysis

Complex numbers are used in signal analysis and other fields as a convenient description for periodically varying signals. The absolute value |z| is interpreted as the amplitude and the argument arg(z) as the phase of a sine wave of given frequency.

If Fourier analysis is employed to write a given real-valued signal as a sum of periodic functions, these periodic functions are often written as complex valued functions of the form

${\displaystyle f(t)=ze^{i\omega t}\,}$ 

where ω represents the angular frequency and the complex number z encodes the phase and amplitude as explained above.

In electrical engineering, the Fourier transform is used to analyze varying voltages and currents. The treatment of resistors, capacitors, and inductors can then be unified by introducing imaginary, frequency-dependent resistances for the latter two and combining all three in a single complex number called the impedance. (Electrical engineers and some physicists use the letter j for the imaginary unit since i is typically reserved for varying currents and may come into conflict with i.) This use is also extended into digital signal processing and digital image processing, which utilize digital versions of Fourier analysis (and Wavelet analysis) to transmit, compress, restore, and otherwise process digital audio signals, still images, and video signals.

Frequency (spectral) ___domain electromagnetism

Maxwell's equations are (usually) written in terms of real vector function's of space and time. When fourier transformed to functions of space and frequency the fields become complex, as in signal analysis. The majority of electromagnetic calculations are done in the frequency ___domain.

Sign convention

In such calculations, engineers tend to use ${\displaystyle e^{j(\omega t-kx)}\,}$  to describe a plane wave, for compatibility with signal analysis, while physicists tend to use ${\displaystyle e^{i(kx-\omega t)}\,}$  for compatibility with quantum mechanics and other scattering calculations. It is therefore sometimes said that j = –i.

Improper integrals

In applied fields, the use of complex analysis is often used to compute certain real-valued improper integrals, by means of complex-valued functions. Several methods exist to do this, see methods of contour integration.

Quantum mechanics

The complex number field is also of utmost importance in quantum mechanics since the underlying theory is built on (infinite dimensional) Hilbert spaces over C. The more limited original formulations of Schrödinger and Heisenberg are also in terms of complex numbers.

Relativity

In special and general relativity, some formulae for the metric on spacetime become simpler if one takes the time variable to be imaginary.

Applied mathematics

In differential equations, it is common to first find all complex roots r of the characteristic equation of a linear differential equation and then attempt to solve the system in terms of base functions of the form f(t) = e^rt.

Fluid dynamics

In fluid dynamics, complex functions are used to describe potential flow in 2d.

Fractals

Certain fractals are plotted in the complex plane e.g. Mandelbrot set and Julia set.

History

The earliest fleeting reference to square roots of negative numbers perhaps occurred in the work of the Greek mathematician and inventor Heron of Alexandria in the 1st century CE, when he considered the volume of an impossible frustum of a pyramid ^[1], though negative numbers were not conceived in the Hellenistic world.

Complex numbers became more prominent in the 16th century, when closed formulas for the roots of cubic and quartic polynomials were discovered by Italian mathematicians (see Niccolo Fontana Tartaglia, Gerolamo Cardano). It was soon realized that these formulas, even if one was only interested in real solutions, sometimes required the manipulation of square roots of negative numbers. For example, Tartaglia's cubic formula gives the following solution to the equation ${\displaystyle x^{3}-x=0}$ :

${\displaystyle {\frac {1}{\sqrt {3}}}\left({\sqrt {-1}}^{1/3}+{\frac {1}{{\sqrt {-1}}^{1/3}}}\right).}$ 

At first glance this looks like nonsense. However formal calculations with complex numbers show that the equation ${\displaystyle z^{3}=i}$  has solutions −i, ${\displaystyle {\frac {\sqrt {3}}{2}}+{\frac {1}{2}}i}$  and ${\displaystyle {\frac {-{\sqrt {3}}}{2}}+{\frac {1}{2}}i}$ . Substituting these in turn for ${\displaystyle {\sqrt {-1}}^{1/3}}$  into the cubic formula and simplifying, one gets 0, 1 and −1 as the solutions of ${\displaystyle x^{3}-x=0.}$ 

This was doubly unsettling since not even negative numbers were considered to be on firm ground at the time. The term "imaginary" for these quantities was coined by René Descartes in 1637 and was meant to be derogatory (see imaginary number for a discussion of the "reality" of complex numbers). A further source of confusion was that the equation ${\displaystyle {\sqrt {-1}}^{2}={\sqrt {-1}}{\sqrt {-1}}=-1}$  seemed to be capriciously inconsistent with the algebraic identity ${\displaystyle {\sqrt {a}}{\sqrt {b}}={\sqrt {ab}}}$ , which is valid for positive real numbers a and b, and which was also used in complex number calculations with one of a, b positive and the other negative. The incorrect use of this identity (and the related identity ${\displaystyle {\frac {1}{\sqrt {a}}}={\sqrt {\frac {1}{a}}}}$ ) in the case when both a and b are negative even bedeviled Euler. This difficulty eventually led to the convention of using the special symbol i in place of ${\displaystyle {\sqrt {-1}}}$  to guard against this mistake.

The 18th century saw the labors of Abraham de Moivre and Leonhard Euler. To De Moivre is due (1730) the well-known formula which bears his name, de Moivre's formula:

${\displaystyle (\cos \theta +i\sin \theta )^{n}=\cos n\theta +i\sin n\theta \,}$ 

and to Euler (1748) Euler's formula of complex analysis:

${\displaystyle \cos \theta +i\sin \theta =e^{i\theta }.\,}$ 

The existence of complex numbers was not completely accepted until the geometrical interpretation (see below) had been described by Caspar Wessel in 1799; it was rediscovered several years later and popularized by Carl Friedrich Gauss, and as a result the theory of complex numbers received a notable expansion. The idea of the graphic representation of complex numbers had appeared, however, as early as 1685, in Wallis's De Algebra tractatus.

Wessel's memoir appeared in the Proceedings of the Copenhagen Academy for 1799, and is exceedingly clear and complete, even in comparison with modern works. He also considers the sphere, and gives a quaternion theory from which he develops a complete spherical trigonometry. In 1804 the Abbé Buée independently came upon the same idea which Wallis had suggested, that ${\displaystyle \pm {\sqrt {-1}}}$  should represent a unit line, and its negative, perpendicular to the real axis. Buée's paper was not published until 1806, in which year Jean-Robert Argand also issued a pamphlet on the same subject. It is to Argand's essay that the scientific foundation for the graphic representation of complex numbers is now generally referred. Nevertheless, in 1831 Gauss found the theory quite unknown, and in 1832 published his chief memoir on the subject, thus bringing it prominently before the mathematical world. Mention should also be made of an excellent little treatise by Mourey (1828), in which the foundations for the theory of directional numbers are scientifically laid. The general acceptance of the theory is not a little due to the labors of Augustin Louis Cauchy and Niels Henrik Abel, and especially the latter, who was the first to boldly use complex numbers with a success that is well known.

The common terms used in the theory are chiefly due to the founders. Argand called ${\displaystyle \cos \phi +i\sin \phi }$  the direction factor, and ${\displaystyle r={\sqrt {a^{2}+b^{2}}}}$  the modulus; Cauchy (1828) called ${\displaystyle \cos \phi +i\sin \phi }$  the reduced form (l'expression réduite); Gauss used i for ${\displaystyle {\sqrt {-1}}}$ , introduced the term complex number for ${\displaystyle a+bi}$ , and called ${\displaystyle a^{2}+b^{2}}$  the norm.

The expression direction coefficient, often used for ${\displaystyle \cos \phi +i\sin \phi }$ , is due to Hankel (1867), and absolute value, for modulus, is due to Weierstrass.

Following Cauchy and Gauss have come a number of contributors of high rank, of whom the following may be especially mentioned: Kummer (1844), Leopold Kronecker (1845), Scheffler (1845, 1851, 1880), Bellavitis (1835, 1852), Peacock (1845), and De Morgan (1849). Möbius must also be mentioned for his numerous memoirs on the geometric applications of complex numbers, and Dirichlet for the expansion of the theory to include primes, congruences, reciprocity, etc., as in the case of real numbers.

A complex ring or field is a set of complex numbers which is closed under addition, subtraction, and multiplication. Gauss studied complex numbers of the form ${\displaystyle a+bi}$ , where a and b are integral, or rational (and i is one of the two roots of ${\displaystyle x^{2}+1=0}$ ). His student, Ferdinand Eisenstein, studied the type ${\displaystyle a+b\omega }$ , where ${\displaystyle \omega }$  is a complex root of ${\displaystyle x^{3}-1=0}$ . Other such classes (called cyclotomic fields) of complex numbers are derived from the roots of unity ${\displaystyle x^{k}-1=0}$  for higher values of ${\displaystyle k}$ . This generalization is largely due to Kummer, who also invented ideal numbers, which were expressed as geometrical entities by Felix Klein in 1893. The general theory of fields was created by Évariste Galois, who studied the fields generated by the roots of any polynomial equation

${\displaystyle \ F(x)=0.}$ 

The late writers (from 1884) on the general theory include Weierstrass, Schwarz, Richard Dedekind, Otto Hölder, Berloty, Henri Poincaré, Eduard Study, and Alexander MacFarlane.

The formally correct definition using pairs of real numbers was given in the 19th century.

See also

• Circular motion#Using complex numbers
• Complex geometry
• De Moivre's formula
• Euler's identity
• Hypercomplex number
• Leonhard Euler
• Local field
• Mandelbrot set
• Quaternion
• Riemann sphere (extended complex plane)
• Split-complex number
• Imaginary number/Imaginary unit

Further reading

• An Imaginary Tale: The Story of ${\displaystyle {\sqrt {-1}}}$ , by Paul J. Nahin; Princeton University Press; ISBN 0-691-02795-1 (hardcover, 1998). A gentle introduction to the history of complex numbers and the beginnings of complex analysis.
• Numbers, by H.-D. Ebbinghaus, H. Hermes, F. Hirzebruch, M. Koecher, K. Mainzer, J. Neukirch, A. Prestel, R. Remmert; Springer; ISBN 0-387-97497-0 (hardcover, 1991). An advanced perspective on the historical development of the concept of number.
• The Road to Reality: A Complete Guide to the Laws of the Universe, by Roger Penrose; Alfred A. Knopf, 2005; ISBN 0-679-45443-8. Chapters 4-7 in particular deal extensively (and enthusiastically) with complex numbers.

References

1. http://people.bath.ac.uk/aab20/complexnumbers.html

External links

• John and Betty's Journey Through Complex Numbers
• Weisstein, Eric W. "Complex Number". MathWorld.
• SOS Math - Complex Variables
• Algebraic Structure of Complex Numbers from cut-the-knot
• A history of complex numbers.