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Complex Numbers



Key Terms
The Imaginary Unit (ii) and Pure Imaginary Numbers
Algebraic Representation and the Complex Conjugate
Arithmetic Operations: Calculating in C\mathbb{C}
Geometric Representation: The Complex Plane
The Mathematical Theory: Complex Numbers as a Field
Trigonometric Representation: Radius and Argument
Exponential Form and Euler's Formula
De Moivre's Theorem and nn-th Order Roots
Complex Equations and Polynomial Theory
The Three Forms at a Glance



The Evolution of Numbers: From Counting to Complexity


Mathematics began with natural numbers (N\mathbb{N}) for counting, but as we attempted to solve more equations, we had to "expand the bag". We added whole numbers (Z\mathbb{Z}) for negatives, rational numbers (Q\mathbb{Q}) for quotients, and real numbers (R\mathbb{R}) for values like 2\sqrt{2}. However, the "R-bag" failed to solve the equation x2+1=0x^2 + 1 = 0, leading to the birth of the Complex Numbers (C\mathbb{C}).

Key Terms

Foundations

Complex Number— a number of the form z=a+biz = a + bi
Imaginary Unit— the number ii satisfying i2=−1i^2 = -1
Imaginary Number— a number of the form bibi with b≠0b \neq 0
Pure Imaginary Number— a complex number with zero real part
Real Part— the component aa in z=a+biz = a + bi
Imaginary Part— the coefficient bb in z=a+biz = a + bi
Algebraic Form— the standard representation z=a+biz = a + bi

Representations

Complex Plane— the two-dimensional plane for visualizing complex numbers
Modulus— the distance ∣z∣|z| from the origin
Argument— the angle θ\theta from the positive real axis
Trigonometric Form— representation as r(cos⁡θ+isin⁡θ)r(\cos\theta + i\sin\theta)
Exponential Form— representation as reiθre^{i\theta}

Operations & Structure

Complex Conjugate— the reflection z‾=a−bi\overline{z} = a - bi
Additive Inverse— the number −z-z that sums to zero
Multiplicative Inverse— the number z−1z^{-1} that multiplies to one
Roots of Unity— solutions to zn=1z^n = 1

See All Complex Numbers Definitions →


The Imaginary Unit (ii) and Pure Imaginary Numbers

The real number system, despite its vastness, cannot solve every equation. The simple polynomial x2+1=0x^2 + 1 = 0 has no solution among real numbers because no real number squared produces a negative result. This limitation led mathematicians to define a new number: the imaginary unit ii, where i2=−1i^2 = -1.

From this single definition, an entire class of numbers emerges. Pure imaginary numbers take the form bibi where bb is real — examples include 3i3i, −7i-7i, and 12i\frac{1}{2}i. These numbers exist on their own axis, perpendicular to the real number line.

The powers of ii follow a cyclic pattern that repeats every four steps: i1=ii^1 = i, i2=−1i^2 = -1, i3=−ii^3 = -i, i4=1i^4 = 1, then back to ii. This cycle provides a shortcut for computing any power of ii — simply divide the exponent by 4 and use the remainder.
n mod 4 in Examples
0 1 i0 = 1,   i4 = 1,   i8 = 1
1 i i1 = i,   i5 = i,   i9 = i
2 −1 i2 = −1,   i6 = −1,   i10 = −1
3 −i i3 = −i,   i7 = −i,   i11 = −i
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Algebraic Representation and the Complex Conjugate

Every complex number can be written in algebraic form (also called standard form):

z=a+biz = a + bi


The variables aa and bb represent ordinary real numbers, while ii is the imaginary unit satisfying:

i2=−1i^2 = -1


The letters aa and bb are conventional choices — using xx and yy or any other symbols works equally well.

Mathematicians assigned names to these components. The value aa is called the real part, and the value bb is called the imaginary part:

Re(z)=aandIm(z)=bRe(z) = a \quad \text{and} \quad Im(z) = b


Note that the imaginary part is bb itself — a real number — not bibi. The term "imaginary part" indicates which component of the number the value describes, not the nature of bb as a quantity.

This representation provides the foundation for arithmetic, comparison, and manipulation of complex numbers. For a complete treatment of the algebraic form, including equality conditions, conjugate identities, and classification theorems, see the dedicated page.

For any z=a+biz = a + bi, we construct its conjugate by flipping the sign in front of bb: thus zˉ=a−bi\bar{z} = a - bi. What does this operation actually do? On the complex plane, it mirrors the point through the horizontal axis. Studying this picture makes the abstract definition concrete — the conjugate sits at equal distance from the real axis, but on the opposite side.

Why does this matter? When zz meets zˉ\bar{z} through multiplication, something special occurs: z⋅zˉ=a2+b2z \cdot \bar{z} = a^2 + b^2, a value guaranteed to be real and non-negative. From here, two tests emerge for classification: if z=zˉz = \bar{z}, the number must be real; if zˉ=−z\bar{z} = -z, it must be pure imaginary.

The conjugate operation carries deep significance throughout complex mathematics. It enables division by eliminating imaginary numbers from denominators. It connects directly to the modulus through the identity ∣z∣2=z⋅zˉ|z|^2 = z \cdot \bar{z}. It preserves arithmetic structure: the conjugate of a sum equals the sum of conjugates, and the conjugate of a product equals the product of conjugates. These properties make the conjugate indispensable for proofs, simplifications, and computations across every branch of complex analysis. A dedicated section explores the conjugate in full detail — its properties, its geometric meaning, and its applications to solving equations and simplifying expressions.
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Arithmetic Operations: Calculating in C\mathbb{C}

The four basic operations extend naturally from real numbers into the complex field, though each carries its own character. Two complex numbers are considered equal only when both their real parts match and their imaginary parts match — the equation a+bi=c+dia + bi = c + di demands a=ca = c and b=db = d simultaneously.

Addition and subtraction work exactly as intuition suggests: combine the real components together, combine the imaginary components together. Given z1=3+2iz_1 = 3 + 2i and z2=1−5iz_2 = 1 - 5i, their sum becomes (3+1)+(2−5)i=4−3i(3 + 1) + (2 - 5)i = 4 - 3i. No new techniques required — just careful bookkeeping of like terms.

Multiplication demands more attention. We apply the distributive property as with any binomial product, but one crucial substitution changes everything: wherever i2i^2 appears, we replace it with −1-1. The product (2+3i)(4−i)(2 + 3i)(4 - i) expands to 8−2i+12i−3i28 - 2i + 12i - 3i^2, and since i2=−1i^2 = -1, this simplifies to 8+10i+3=11+10i8 + 10i + 3 = 11 + 10i.

Division presents the only genuine challenge. A complex number in the denominator violates standard mathematical convention — we need real denominators for final answers. The solution employs the conjugate: multiply both numerator and denominator by the conjugate of the denominator. This works because z⋅zˉz \cdot \bar{z} always produces a real number. The fraction 3+2i1−4i\frac{3 + 2i}{1 - 4i} transforms through multiplication by 1+4i1+4i\frac{1 + 4i}{1 + 4i}, converting the denominator to the real value 1+16=171 + 16 = 17 and allowing us to express the result in standard a+bia + bi form.
Operation Rule Example
Addition add real parts, add imaginary parts separately (3 + 2i) + (1 − 5i) = 4 − 3i
Subtraction subtract real parts, subtract imaginary parts separately (3 + 2i) − (1 − 5i) = 2 + 7i
Multiplication distribute (FOIL), then replace i2 with −1 (2 + 3i)(4 − i) = 11 + 10i
Division multiply numerator and denominator by the conjugate of the denominator (3 + 2i) ⁄ (1 − 4i) = (−5 + 14i) ⁄ 17
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Geometric Representation: The Complex Plane

Numbers gain new meaning when we can see them. Every complex number z=a+biz = a + bi corresponds to a unique point (a,b)(a, b) on a two-dimensional plane. The horizontal axis carries the real numbers, the vertical axis carries the pure imaginary numbers, and every other complex number occupies the space between. This visualization, called the complex plane or Argand diagram, transforms abstract algebra into concrete geometry.

The placement of familiar numbers becomes immediate: real numbers like 33 or −2-2 sit directly on the horizontal axis, pure imaginary numbers like 4i4i or −i-i sit on the vertical axis, and the origin represents zero — the only number belonging to both categories simultaneously. A number like 3+2i3 + 2i appears at the point three units right and two units up from the origin.

This geometric perspective reveals structure invisible in pure algebra. The conjugate zˉ\bar{z} appears as a reflection across the real axis. Addition of complex numbers follows the parallelogram rule familiar from vector mathematics. The modulus ∣z∣=a2+b2|z| = \sqrt{a^2 + b^2} measures the straight-line distance from the origin to the point — a direct application of the Pythagorean theorem.

One fundamental difference separates C\mathbb{C} from R\mathbb{R}: complex numbers cannot be ordered. We cannot say 3+2i3 + 2i is greater than 1+5i1 + 5i in any meaningful way. The real line has direction — numbers increase as we move right — but the complex plane extends in all directions equally. Only the modulus provides a sense of size, measuring how far each number lies from the origin.

The Triangle Inequality governs how distances combine: ∣z1+z2∣≤∣z1∣+∣z2∣|z_1 + z_2| \leq |z_1| + |z_2|. Geometrically, this says the direct path between two points never exceeds the detour through the origin. This inequality appears constantly in analysis and provides essential bounds for estimation and proof.
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The Mathematical Theory: Complex Numbers as a Field

Mathematics demands rigor beyond intuition. Before accepting C\mathbb{C} as a legitimate number system, we must verify that it satisfies the structural requirements of a mathematical field. A field is a set equipped with addition and multiplication operations that obey eleven specific axioms — the same rules that govern the rational and real numbers.

The verification proceeds systematically. Addition in C\mathbb{C} is commutative: z1+z2=z2+z1z_1 + z_2 = z_2 + z_1 for any pair of complex numbers. It is also associative: (z1+z2)+z3=z1+(z2+z3)(z_1 + z_2) + z_3 = z_1 + (z_2 + z_3). An additive identity exists — the number 0+0i0 + 0i satisfies z+0=zz + 0 = z for every zz. Every complex number possesses an additive inverse: the negative −z=−a−bi-z = -a - bi fulfills z+(−z)=0z + (-z) = 0.

Multiplication follows parallel rules. The operation is commutative and associative. The multiplicative identity 1+0i1 + 0i satisfies z⋅1=zz \cdot 1 = z. Every nonzero complex number has a multiplicative inverse z−1z^{-1} such that z⋅z−1=1z \cdot z^{-1} = 1. The formula for this inverse employs the conjugate: z−1=zˉ∣z∣2=a−bia2+b2z^{-1} = \frac{\bar{z}}{|z|^2} = \frac{a - bi}{a^2 + b^2}.

The distributive law bridges addition and multiplication: z1⋅(z2+z3)=z1⋅z2+z1⋅z3z_1 \cdot (z_2 + z_3) = z_1 \cdot z_2 + z_1 \cdot z_3. With all eleven axioms confirmed, C\mathbb{C} earns its status as a field — an algebraic structure where arithmetic behaves predictably and division by any nonzero element remains possible.

Yet C\mathbb{C} differs from R\mathbb{R} in one critical respect: it lacks order. No definition of "less than" or "greater than" extends consistently to complex numbers. We cannot arrange them on a line from smallest to largest. The complex field trades this linear ordering for something arguably more powerful — algebraic completeness, where every polynomial equation finds its solutions.

Trigonometric Representation: Radius and Argument

The algebraic form z=a+biz = a + bi serves well for addition and subtraction, but multiplication and powers become unwieldy as expressions grow complex. A different representation emerges from the geometric perspective: instead of describing where a point lies using horizontal and vertical coordinates, we specify how far it sits from the origin and what angle it makes with the positive real axis.

The distance from the origin is the modulus r=∣z∣=a2+b2r = |z| = \sqrt{a^2 + b^2}, computed through the Pythagorean theorem. The angle θ\theta, called the argument and written arg⁡(z)\arg(z), measures the counterclockwise rotation from the positive real axis to the line connecting the origin to zz. Together, these two values determine the number completely.

The connection between forms follows from basic trigonometry. If a point lies at distance rr and angle θ\theta, its horizontal coordinate equals rcos⁡θr\cos\theta and its vertical coordinate equals rsin⁡θr\sin\theta. Thus z=rcos⁡θ+irsin⁡θz = r\cos\theta + ir\sin\theta, which factors as z=r(cos⁡θ+isin⁡θ)z = r(\cos\theta + i\sin\theta). Mathematicians abbreviate this as z=rcisθz = r\text{cis}\theta, where "cis" stands for "cosine plus i sine."

Converting from algebraic to trigonometric form requires finding both rr and θ\theta. The modulus comes directly from r=a2+b2r = \sqrt{a^2 + b^2}. The argument requires more care: while tan⁡θ=ba\tan\theta = \frac{b}{a} provides a starting point, the actual quadrant of zz determines whether adjustments are needed. A number in the second quadrant has a different argument than one in the fourth, even if both share the same tangent value.

Certain angles deserve memorization. Numbers on the positive real axis have argument 0°0°. Pure positive imaginaries point straight up at 90°90°. Negative reals aim backward at 180°180°. Negative imaginaries point downward at 270°270° or equivalently −90°-90°. Recognizing these special cases accelerates computation and builds geometric intuition for how arguments behave.

The true power of trigonometric form reveals itself in multiplication and division. When two numbers multiply, their moduli multiply and their arguments add: z1⋅z2=r1r2cis(θ1+θ2)z_1 \cdot z_2 = r_1 r_2 \text{cis}(\theta_1 + \theta_2). Division works inversely — moduli divide, arguments subtract. These elegant rules transform tedious algebraic expansion into simple arithmetic on lengths and angles.
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Exponential Form and Euler's Formula

The trigonometric form rcisθr\text{cis}\theta simplifies multiplication beautifully, but an even more elegant representation exists. In the eighteenth century, Leonhard Euler discovered a remarkable connection between exponential functions and trigonometry that would reshape mathematics forever: the identity eiθ=cos⁡θ+isin⁡θe^{i\theta} = \cos\theta + i\sin\theta.

This equation appears almost magical at first glance. The left side involves raising a real number to an imaginary power — an operation with no obvious meaning. Yet when we extend the exponential function into the complex domain through its infinite series expansion, the formula emerges naturally. The real parts collect to form cosine, the imaginary parts assemble into sine, and the equality holds for every real angle θ\theta.

From Euler's formula flows the exponential form of a complex number: z=reiθz = re^{i\theta}, where rr is the modulus and θ\theta is the argument. This notation is mathematically equivalent to rcisθr\text{cis}\theta but carries computational advantages. Standard exponent rules now apply directly to complex arithmetic. Multiplication becomes z1⋅z2=r1r2ei(θ1+θ2)z_1 \cdot z_2 = r_1 r_2 e^{i(\theta_1 + \theta_2)} — we simply multiply the coefficients and add the exponents. Division follows as z1z2=r1r2ei(θ1−θ2)\frac{z_1}{z_2} = \frac{r_1}{r_2}e^{i(\theta_1 - \theta_2)}.

Powers reduce to almost trivial calculation. Raising z=reiθz = re^{i\theta} to the nn-th power yields zn=rneinθz^n = r^n e^{in\theta}. No binomial expansion, no collecting terms, no tracking powers of ii — just raise the modulus to the power and multiply the argument by nn. This efficiency becomes indispensable when computing high powers or extracting roots.

One specialization of Euler's formula achieves legendary status among mathematicians. Setting θ=π\theta = \pi produces eiπ=cos⁡π+isin⁡π=−1e^{i\pi} = \cos\pi + i\sin\pi = -1, which rearranges to eiπ+1=0e^{i\pi} + 1 = 0. This single equation — known as Euler's Identity — connects five of mathematics' most fundamental constants: ee, ii, π\pi, 11, and 00. It unites exponential growth, imaginary numbers, circular geometry, multiplicative identity, and additive identity into one compact statement, often celebrated as the most beautiful formula in all of mathematics.
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De Moivre's Theorem and nn-th Order Roots

The exponential form hints at a powerful pattern: raising z=rcisθz = r\text{cis}\theta to the nn-th power multiplies the argument by nn while raising the modulus to that power. Abraham de Moivre formalized this observation into the theorem bearing his name: (cos⁡θ+isin⁡θ)n=cos⁡(nθ)+isin⁡(nθ)(\cos\theta + i\sin\theta)^n = \cos(n\theta) + i\sin(n\theta) for any integer nn. In compact notation, (cisθ)n=cis(nθ)(\text{cis}\theta)^n = \text{cis}(n\theta).

The practical formula extends to any complex number in trigonometric form: zn=rncis(nθ)z^n = r^n\text{cis}(n\theta). Computing (1+i)20(1 + i)^{20} through algebraic expansion would require enormous effort, but De Moivre's theorem reduces it to routine calculation. First convert: 1+i=2cis(45°)1 + i = \sqrt{2}\text{cis}(45°). Then apply the formula: (2)20cis(20⋅45°)=1024cis(900°)=1024cis(180°)=−1024(\sqrt{2})^{20}\text{cis}(20 \cdot 45°) = 1024\text{cis}(900°) = 1024\text{cis}(180°) = -1024. What could have been pages of algebra becomes three lines.

The theorem's true depth emerges when we reverse direction and seek roots rather than powers. Given a complex number ww, what values of zz satisfy zn=wz^n = w? The answer reveals geometric elegance: exactly nn distinct roots exist, and they distribute themselves evenly around a circle centered at the origin.

If w=Rcisϕw = R\text{cis}\phi, the nn-th roots follow the formula zk=R1/ncis(ϕ+360°kn)z_k = R^{1/n}\text{cis}\left(\frac{\phi + 360°k}{n}\right) for k=0,1,2,…,n−1k = 0, 1, 2, \ldots, n-1. Each root shares the same modulus R1/nR^{1/n}, placing all of them on a circle of that radius. Their arguments differ by exactly 360°n\frac{360°}{n}, spacing them uniformly like vertices of a regular polygon.

The roots of unity — solutions to zn=1z^n = 1 — deserve special attention. Since 1=1cis(0°)1 = 1\text{cis}(0°), the nn-th roots of unity are zk=cis(360°kn)z_k = \text{cis}\left(\frac{360°k}{n}\right). For n=4n = 4, the fourth roots of unity are 11, ii, −1-1, and −i-i, forming a square on the unit circle. For n=6n = 6, we obtain a regular hexagon. These roots appear throughout mathematics: in Fourier analysis, in polynomial factorization, in signal processing, and in the study of symmetry. The primitive root ω=cis(360°n)\omega = \text{cis}\left(\frac{360°}{n}\right) generates all others as powers: ω0,ω1,ω2,…,ωn−1\omega^0, \omega^1, \omega^2, \ldots, \omega^{n-1}.
n n-th roots of unity Geometric figure on the unit circle Primitive root ω
2 1,   −1 two opposite points on the real axis −1
3 1,   cis(120°),   cis(240°) equilateral triangle cis(120°)
4 1,   i,   −1,   −i square i
6 1,   cis(60°),   cis(120°),   −1,   cis(240°),   cis(300°) regular hexagon cis(60°)
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Complex Equations and Polynomial Theory

The complex number system achieves what no smaller number system can: it provides a home for every polynomial root. This completeness, formalized in the Fundamental Theorem of Algebra, states that every non-constant polynomial with complex coefficients possesses at least one complex root. The immediate consequence transforms our understanding of polynomials: a polynomial of degree nn factors completely into exactly nn linear terms over C\mathbb{C}, meaning it has precisely nn roots when counted with multiplicity.

Consider the equation x4+1=0x^4 + 1 = 0, which has no real solutions since x4x^4 is always non-negative. In C\mathbb{C}, four distinct roots exist, evenly spaced around the unit circle at arguments 45°45°, 135°135°, 225°225°, and 315°315°. The geometric representation reveals their symmetric arrangement, while De Moivre's theorem provides the computational machinery to find them.

Vieta's formulas establish a profound connection between a polynomial's roots and its coefficients. For a quadratic z2+bz+c=0z^2 + bz + c = 0 with roots z1z_1 and z2z_2, the sum of roots equals −b-b and the product equals cc. These relationships extend to higher degrees: for a cubic, the sum of roots, the sum of pairwise products, and the product of all three roots each correspond to specific coefficients. This bridge between algebra and the structure of solutions enables problem-solving techniques unavailable through direct calculation.

Polynomials with real coefficients exhibit special behavior: their non-real roots always appear in conjugate pairs. If z0z_0 satisfies a real polynomial, so does z0ˉ\bar{z_0}. This symmetry explains why real quadratics either have two real roots or two complex conjugates — never one of each. The geometric interpretation places conjugate pairs as reflections across the real axis, preserving the polynomial's real character.

Certain equations showcase the interplay between conjugation and powers. The equation zn=zˉz^n = \bar{z} yields exactly n+2n + 2 solutions: zero itself, plus n+1n + 1 values distributed on the unit circle. Solving such equations requires translating into trigonometric form, applying the constraint that rneinθ=re−iθr^n e^{in\theta} = re^{-i\theta}, and extracting the permissible values of rr and θ\theta. These techniques, combining algebraic manipulation with geometric insight, represent the full power of complex number theory applied to equation solving.
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The Three Forms at a Glance

The sections above introduce three equivalent ways of writing a complex number — algebraic, trigonometric, and exponential. Each excels at a different kind of computation. The table below collects them side by side: how each form is written, what it is best for, how the major operations look, and how to convert from algebraic form. Use it to choose the right representation before starting a calculation, and to navigate to the dedicated page for each form.
Aspect Algebraic Trigonometric Exponential
Notation z = a + bi z = r(cos θ + i sin θ) = r cis θ z = reiθ
Best for addition, subtraction, equality, real/imaginary part extraction multiplication, division, geometric reasoning powers, roots, calculus, theoretical work
Multiplication z1·z2 distribute and substitute i2 = −1 r1r2 cis(θ1 + θ2) r1r2 ei(θ1+θ2)
Powers zn binomial expansion — tedious for large n rn cis(nθ)   (De Moivre) rn einθ
Conversion from algebraic already standard form r = √(a² + b²),   θ = arg(z) (mind the quadrant) same r and θ as trigonometric form