In number theory, a cyclotomic field is a number field obtained by adjoining a complex root of unity to Q, the field of rational numbers.

Cyclotomic fields played a crucial role in the development of modern algebra and number theory because of their relation with Fermat's Last Theorem. It was in the process of his deep investigations of the arithmetic of these fields (for prime n) – and more precisely, because of the failure of unique factorization in their rings of integers – that Ernst Kummer first introduced the concept of an ideal number and proved his celebrated congruences.

Definition

For n ≥ 1, let ζ_n = e^2πi/n ∈ C; this is a primitive nth root of unity. Then the nth cyclotomic field is the extension Q(ζ_n) of Q generated by ζ_n.

Properties

• The nth cyclotomic polynomial

${\displaystyle \Phi _{n}(x)=\prod _{\stackrel {1\leq k\leq n}{\gcd(k,n)=1))\!\!\!\left(x-e^{2\pi ik/n}\right)=\prod _{\stackrel {1\leq k\leq n}{\gcd(k,n)=1))\!\!\!(x-{\zeta _{n))^{k})}$

is irreducible, so it is the minimal polynomial of ζ_n over Q.

• The conjugates of ζ_n in C are therefore the other primitive nth roots of unity: ζ^k
  _n for 1 ≤ k ≤ n with gcd(k, n) = 1.
• The degree of Q(ζ_n) is therefore [Q(ζ_n) : Q] = deg Φ_n = φ(n), where φ is Euler's totient function.
• The roots of xⁿ − 1 are the powers of ζ_n, so Q(ζ_n) is the splitting field of xⁿ − 1 (or of Φ(x)) over Q.
• Therefore Q(ζ_n) is a Galois extension of Q.
• The Galois group ${\displaystyle \operatorname {Gal} (\mathbf {Q} (\zeta _{n})/\mathbf {Q} )}$ is naturally isomorphic to the multiplicative group ${\displaystyle (\mathbf {Z} /n\mathbf {Z} )^{\times ))$, which consists of the invertible residues modulo n, which are the residues a mod n with 1 ≤ a ≤ n and gcd(a, n) = 1. The isomorphism sends each ${\displaystyle \sigma \in \operatorname {Gal} (\mathbf {Q} (\zeta _{n})/\mathbf {Q} )}$ to a mod n, where a is an integer such that σ(ζ_n) = ζ^a
  _n.
• The ring of integers of Q(ζ_n) is Z[ζ_n].
• For n > 2, the discriminant of the extension Q(ζ_n) / Q is^[1]

${\displaystyle (-1)^{\varphi (n)/2}\,{\frac {n^{\varphi (n))){\displaystyle \prod _{p|n}p^{\varphi (n)/(p-1)))}.}$

• In particular, Q(ζ_n) / Q is unramified above every prime not dividing n.
• If n is a power of a prime p, then Q(ζ_n) / Q is totally ramified above p.
• If q is a prime not dividing n, then the Frobenius element ${\displaystyle \operatorname {Frob} _{q}\in \operatorname {Gal} (\mathbf {Q} (\zeta _{n})/\mathbf {Q} )}$ corresponds to the residue of q in ${\displaystyle (\mathbf {Z} /n\mathbf {Z} )^{\times ))$.
• The group of roots of unity in Q(ζ_n) has order n or 2n, according to whether n is even or odd.
• The unit group Z[ζ_n]^× is a finitely generated abelian group of rank φ(n)/2 – 1, for any n > 2, by the Dirichlet unit theorem. In particular, Z[ζ_n]^× is finite only for n ∈ {1, 2, 3, 4, 6}. The torsion subgroup of Z[ζ_n]^× is the group of roots of unity in Q(ζ_n), which was described in the previous item. Cyclotomic units form an explicit finite-index subgroup of Z[ζ_n]^×.
• The Kronecker–Weber theorem states that every finite abelian extension of Q in C is contained in Q(ζ_n) for some n. Equivalently, the union of all the cyclotomic fields Q(ζ_n) is the maximal abelian extension Q^ab of Q.

Relation with regular polygons

Gauss made early inroads in the theory of cyclotomic fields, in connection with the problem of constructing a regular n-gon with a compass and straightedge. His surprising result that had escaped his predecessors was that a regular 17-gon could be so constructed. More generally, for any integer n ≥ 3, the following are equivalent:

• a regular n-gon is constructible;
• there is a sequence of fields, starting with Q and ending with Q(ζ_n), such that each is a quadratic extension of the previous field;
• φ(n) is a power of 2;
• ${\displaystyle n=2^{a}p_{1}\cdots p_{r))$ for some integers a, r ≥ 0 and Fermat primes ${\displaystyle p_{1},\ldots ,p_{r))$. (A Fermat prime is an odd prime p such that p − 1 is a power of 2. The known Fermat primes are 3, 5, 17, 257, 65537, and it is likely that there are no others.)

Small examples

• n = 3 and n = 6: The equations ${\displaystyle \zeta _{3}={\tfrac {-1+{\sqrt {-3))}{2))}$ and ${\displaystyle \zeta _{6}={\tfrac {1+{\sqrt {-3))}{2))}$ show that Q(ζ₃) = Q(ζ₆) = Q(√−3 ), which is a quadratic extension of Q. Correspondingly, a regular 3-gon and a regular 6-gon are constructible.
• n = 4: Similarly, ζ₄ = i, so Q(ζ₄) = Q(i), and a regular 4-gon is constructible.
• n = 5: The field Q(ζ₅) is not a quadratic extension of Q, but it is a quadratic extension of the quadratic extension Q(√5 ), so a regular 5-gon is constructible.

Relation with Fermat's Last Theorem

A natural approach to proving Fermat's Last Theorem is to factor the binomial xⁿ + yⁿ, where n is an odd prime, appearing in one side of Fermat's equation

${\displaystyle x^{n}+y^{n}=z^{n))$

as follows:

${\displaystyle x^{n}+y^{n}=(x+y)(x+\zeta y)\cdots (x+\zeta ^{n-1}y)}$

Here x and y are ordinary integers, whereas the factors are algebraic integers in the cyclotomic field Q(ζ_n). If unique factorization holds in the cyclotomic integers Z[ζ_n], then it can be used to rule out the existence of nontrivial solutions to Fermat's equation.

Several attempts to tackle Fermat's Last Theorem proceeded along these lines, and both Fermat's proof for n = 4 and Euler's proof for n = 3 can be recast in these terms. The complete list of n for which Q(ζ_n) has unique factorization is^[2]

• 1 through 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 40, 42, 44, 45, 48, 50, 54, 60, 66, 70, 84, 90.

Kummer found a way to deal with the failure of unique factorization. He introduced a replacement for the prime numbers in the cyclotomic integers Z[ζ_n], measured the failure of unique factorization via the class number h_n and proved that if h_p is not divisible by a prime p (such p are called regular primes) then Fermat's theorem is true for the exponent n = p. Furthermore, he gave a criterion to determine which primes are regular, and established Fermat's theorem for all prime exponents p less than 100, except for the irregular primes 37, 59, and 67. Kummer's work on the congruences for the class numbers of cyclotomic fields was generalized in the twentieth century by Iwasawa in Iwasawa theory and by Kubota and Leopoldt in their theory of p-adic zeta functions.

List of class numbers of cyclotomic fields

(sequence A061653 in the OEIS), or OEIS: A055513 or OEIS: A000927 for the ${\displaystyle h}$-part (for prime n)