Root of unity

In mathematics, a root of unity, occasionally called a de Moivre number, is any complex number that yields 1 when raised to some positive integer power n. Roots of unity are used in many branches of mathematics, and are especially important in number theory, the theory of group characters, and the discrete Fourier transform.

Roots of unity can be defined in any field. If the characteristic of the field is zero, the roots are complex numbers that are also algebraic integers. For fields with a positive characteristic, the roots belong to a finite field, and, conversely, every nonzero element of a finite field is a root of unity. Any algebraically closed field contains exactly n nth roots of unity, except when n is a multiple of the (positive) characteristic of the field.

An nth root of unity, where n is a positive integer, is a number z satisfying the equation[1][2]

Unless otherwise specified, the roots of unity may be taken to be complex numbers (including the number 1, and the number –1 if n is even, which are complex with a zero imaginary part), and in this case, the nth roots of unity are

However, the defining equation of roots of unity is meaningful over any field (and even over any ring) F, and this allows considering roots of unity in F. Whichever is the field F, the roots of unity in F are either complex numbers, if the characteristic of F is 0, or, otherwise, belong to a finite field. Conversely, every nonzero element in a finite field is a root of unity in that field. See Root of unity modulo n and Finite field for further details.

An nth root of unity is said to be primitive if it is not an mth root of unity for some smaller m, that is if

If n is a prime number, then all nth roots of unity, except 1, are primitive.

In the above formula in terms of exponential and trigonometric functions, the primitive nth roots of unity are those for which k and n are coprime integers.

Subsequent sections of this article will comply with complex roots of unity. For the case of roots of unity in fields of nonzero characteristic, see Finite field § Roots of unity. For the case of roots of unity in rings of modular integers, see Root of unity modulo n.

Every nth root of unity z is a primitive ath root of unity for some an, which is the smallest positive integer such that za = 1.

Any integer power of an nth root of unity is also an nth root of unity, as

This is also true for negative exponents. In particular, the reciprocal of an nth root of unity is its complex conjugate, and is also an nth root of unity:

If z is an nth root of unity and ab (mod n) then za = zb. Indeed, by the definition of congruence modulo n, a = b + kn for some integer k, and hence

Therefore, given a power za of z, one has za = zr, where 0 ≤ r < n is the remainder of the Euclidean division of a by n.

Let z be a primitive nth root of unity. Then the powers z, z2, ..., zn−1, zn = z0 = 1 are nth roots of unity and are all distinct. (If za = zb where 1 ≤ a < bn, then zba = 1, which would imply that z would not be primitive.) This implies that z, z2, ..., zn−1, zn = z0 = 1 are all of the nth roots of unity, since an nth-degree polynomial equation over a field (in this case the field of complex numbers) has at most n solutions.

Let z be a primitive nth root of unity. A power w = zk of z is a primitive ath root of unity for

Thus, if k and n are coprime, zk is also a primitive nth root of unity, and therefore there are φ(n) distinct primitive nth roots of unity (where φ is Euler's totient function). This implies that if n is a prime number, all the roots except +1 are primitive.

In other words, if R(n) is the set of all nth roots of unity and P(n) is the set of primitive ones, R(n) is a disjoint union of the P(n):

where the notation means that d goes through all the positive divisors of n, including 1 and n.

Since the cardinality of R(n) is n, and that of P(n) is φ(n), this demonstrates the classical formula

The product and the multiplicative inverse of two roots of unity are also roots of unity. In fact, if xm = 1 and yn = 1, then (x−1)m = 1, and (xy)k = 1, where k is the least common multiple of m and n.

Therefore, the roots of unity form an abelian group under multiplication. This group is the torsion subgroup of the circle group.

For an integer n, the product and the multiplicative inverse of two nth roots of unity are also nth roots of unity. Therefore, the nth roots of unity form an abelian group under multiplication.

Given a primitive nth root of unity ω, the other nth roots are powers of ω. This means that the group of the nth roots of unity is a cyclic group. It is worth remarking that the term of cyclic group originated from the fact that this group is a subgroup of the circle group.

If k is an integer, ωk is a primitive nth root of unity if and only if k and n are coprime. In this case, the map

The rules of exponentiation imply that the composition of two such automorphisms is obtained by multiplying the exponents. It follows that the map

This shows that this Galois group is abelian, and implies thus that the primitive roots of unity may be expressed in terms of radicals.

This formula shows that in the complex plane the nth roots of unity are at the vertices of a regular n-sided polygon inscribed in the unit circle, with one vertex at 1 (see the plots for n = 3 and n = 5 on the right.) This geometric fact accounts for the term "cyclotomic" in such phrases as cyclotomic field and cyclotomic polynomial; it is from the Greek roots "cyclo" (circle) plus "tomos" (cut, divide).

which is valid for all real x, can be used to put the formula for the nth roots of unity into the form

The nth roots of unity are, by definition, the roots of the polynomial xn − 1, and are thus algebraic numbers. As this polynomial is not irreducible (except for n = 1), the primitive nth roots of unity are roots of an irreducible polynomial of lower degree, called the nth cyclotomic polynomial, and often denoted Φn. The degree of Φn is given by Euler's totient function, which counts (among other things) the number of primitive nth roots of unity. The roots of Φn are exactly the primitive nth roots of unity.

Gauss proved that a primitive nth root of unity can be expressed using only square roots, addition, subtraction, multiplication and division if and only if it is possible to construct with compass and straightedge the regular n-gon. This is the case if and only if n is either a power of two or the product of a power of two and Fermat primes that are all different.

is n-periodic (because z j + n = z jz n = z j for all values of j), and the n sequences of powers

can be expressed as a linear combination of powers of a primitive nth root of unity:

This is a form of Fourier analysis. If j is a (discrete) time variable, then k is a frequency and Xk is a complex amplitude.

Let SR(n) be the sum of all the nth roots of unity, primitive or not. Then

This is an immediate consequence of Vieta's formulas. In fact, the nth roots of unity being the roots of the polynomial Xn – 1, their sum is the coefficient of degree n – 1, which is either 1 or 0 according whether n = 1 or n > 1.

Alternatively, for n = 1 there is nothing to prove, and for n > 1 there exists a root z ≠ 1 – since the set S of all the nth roots of unity is a group, zS = S, so the sum satisfies z SR(n) = SR(n), whence SR(n) = 0.

In the section Elementary properties, it was shown that if R(n) is the set of all nth roots of unity and P(n) is the set of primitive ones, R(n) is a disjoint union of the P(n):

In this formula, if d < n, then SR( n/d) = 0, and for d = n: SR( n/d) = 1. Therefore, SP(n) = μ(n).

This is the special case cn(1) of Ramanujan's sum cn(s), defined as the sum of the sth powers of the primitive nth roots of unity:

From the summation formula follows an orthogonality relationship: for j = 1, … , n and j′ = 1, … , n

where δ is the Kronecker delta and z is any primitive nth root of unity.

defines a discrete Fourier transform. Computing the inverse transformation using Gaussian elimination requires O(n3) operations. However, it follows from the orthogonality that U is unitary. That is,

and thus the inverse of U is simply the complex conjugate. (This fact was first noted by Gauss when solving the problem of trigonometric interpolation). The straightforward application of U or its inverse to a given vector requires O(n2) operations. The fast Fourier transform algorithms reduces the number of operations further to O(n log n).

are precisely the nth roots of unity, each with multiplicity 1. The nth cyclotomic polynomial is defined by the fact that its zeros are precisely the primitive nth roots of unity, each with multiplicity 1.

where z1, z2, z3, …, zφ(n) are the primitive nth roots of unity, and φ(n) is Euler's totient function. The polynomial Φn(z) has integer coefficients and is an irreducible polynomial over the rational numbers (that is, it cannot be written as the product of two positive-degree polynomials with rational coefficients). The case of prime n, which is easier than the general assertion, follows by applying Eisenstein's criterion to the polynomial

Every nth root of unity is a primitive dth root of unity for exactly one positive divisor d of n. This implies that

This formula represents the factorization of the polynomial zn − 1 into irreducible factors:

where μ is the Möbius function. So the first few cyclotomic polynomials are

If p is a prime number, then all the pth roots of unity except 1 are primitive pth roots, and we have

Substituting any positive integer ≥ 2 for z, this sum becomes a base z repunit. Thus a necessary (but not sufficient) condition for a repunit to be prime is that its length be prime.

Many restrictions are known about the values that cyclotomic polynomials can assume at integer values. For example, if p is prime, then d ∣ Φp(d) if and only d ≡ 1 (mod p).

Cyclotomic polynomials are solvable in radicals, as roots of unity are themselves radicals. Moreover, there exist more informative radical expressions for nth roots of unity with the additional property[4] that every value of the expression obtained by choosing values of the radicals (for example, signs of square roots) is a primitive nth root of unity. This was already shown by Gauss in 1797.[5] Efficient algorithms exist for calculating such expressions.[6]

The nth roots of unity form under multiplication a cyclic group of order n, and in fact these groups comprise all of the finite subgroups of the multiplicative group of the complex number field. A generator for this cyclic group is a primitive nth root of unity.

The nth roots of unity form an irreducible representation of any cyclic group of order n. The orthogonality relationship also follows from group-theoretic principles as described in Character group.

The roots of unity appear as entries of the eigenvectors of any circulant matrix; that is, matrices that are invariant under cyclic shifts, a fact that also follows from group representation theory as a variant of Bloch's theorem.[7] In particular, if a circulant Hermitian matrix is considered (for example, a discretized one-dimensional Laplacian with periodic boundaries[8]), the orthogonality property immediately follows from the usual orthogonality of eigenvectors of Hermitian matrices.

Conversely, every abelian extension of the rationals is such a subfield of a cyclotomic field – this is the content of a theorem of Kronecker, usually called the Kronecker–Weber theorem on the grounds that Weber completed the proof.

In the complex plane, the red points are the fifth roots of unity, and the black points are the sums of a fifth root of unity and its complex conjugate.
In the complex plane, the corners of the two squares are the eighth roots of unity

For four other values of n, the primitive roots of unity are not quadratic integers, but the sum of any root of unity with its complex conjugate (also an nth root of unity) is a quadratic integer.

For n = 5, 10, none of the non-real roots of unity (which satisfy a quartic equation) is a quadratic integer, but the sum z + z = 2 Re z of each root with its complex conjugate (also a 5th root of unity) is an element of the ring Z[ 1 + 5/2] (D = 5). For two pairs of non-real 5th roots of unity these sums are inverse golden ratio and minus golden ratio.

For n = 8, for any root of unity z + z equals to either 0, ±2, or ±2 (D = 2).

For n = 12, for any root of unity, z + z equals to either 0, ±1, ±2 or ±3 (D = 3).