In mathematics, the nth roots of unity or de Moivre numbers, named after Abraham de Moivre (1667 - 1754), are complex numbers located on the unit circle. They form the vertices of a n-sided regular polygon with one vertex on 1.
Definition
For a given n the complex numbers z which solve
are called the nth roots of unity.
There are n different nth roots of unity.
See Exponentiation#powers of one.
The nth roots of unity form a cyclic group of order n under multiplication, with 1 as the identity element.
The generators for this cyclic group is called the primitive nth roots of unity.
Examples
The only first root of unity is
The two square roots of unity are
The only primitive square root of unity is
The three third roots of unity are
The two primitive third roots of unity are
The four fourth roots of unity are
The two primitive fourth roots of unity are
Properties
According to Euler's identity the nth roots of unity can be written
As long as n is at least 2, these numbers add up to 0, a simple fact that is of constant use in mathematics. It is proved by recognising the sum as a geometric progression.
is a direction if is a real number.
is a root of unity if is a rational number.
if is an integer.
The unitary matrix is fundamental for the discrete fourier transform.
For example, for n= 4 the matrix looks like this.
A detailed exposition of the orthogonality relationship is given in the article character group.
The primitive nth roots of unity are precisely the numbers of the form where k and n are coprime. Therefore, there are φ(n) different primitive nth roots of unity, where φ(n) denotes Euler's phi function.
Cyclotomic polynomials
The nth roots of unity are precisely the zeros of the polynomial
The primitive nth roots of unity are precisely the zeros of the nth cyclotomic polynomial
where z1,...,zφ(n) are the primitive nth roots of unity. The polynomial Φn(X) has integer coefficients and is irreducible over the rationals (i.e., cannot be written as a product of two positive-degree polynomials with rational coefficients). The case of prime n, which is easier than the general assertion, follows from Eisenstein's criterion.
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 Xn - 1 into irreducible factors and can also be used to compute the cyclotomic polynomials recursively. The first few are
- Φ1(X) = X − 1
- Φ2(X) = X + 1
- Φ3(X) = X2 + X + 1
- Φ4(X) = X2 + 1
- Φ5(X) = X4 + X3 + X2 + X + 1
- Φ6(X) = X2 − X + 1
In general, if p is a prime number, then all pth roots of unity except 1 are primitive pth roots, and we have
Note that, contrary to first appearances, not all coefficients of all cyclotomic polynomials are 1, −1, or 0; the first polynomial where this occurs is Φ105 where 105=3×5×7 is the first product of three odd primes.
Cyclotomic fields
By adjoining a primitive nth root of unity to Q, one obtains the n th cyclotomic field Fn. This field contains all nth roots of unity and is the splitting field of the nth cyclotomic polynomial over Q. The field extension Fn/Q has degree φ(n) and its Galois group is naturally isomorphic to the multiplicative group of units of the ring Z/nZ.
As the Galois group of Fn/Q is abelian, this is an abelian extension. Every subfield of a cyclotomic field is an abelian extension of the rationals. In these cases Galois theory can be written out quite explicitly in terms of Gaussian periods: this theory from the Disquisitiones Arithmeticae of Gauss was published many years before Galois.
Conversely, every abelian extension of the rationals is such a subfield of a cyclotomic field - a theorem of Kronecker, usually called the Kronecker-Weber theorem on the grounds that Weber supplied the proof.