Cyclotomic polynomials.

For n : ℕ and an integral domain R, we define a modified version of the n-th cyclotomic polynomial with coefficients in R, denoted cyclotomic' n R, as ∏ (X - μ), where μ varies over the primitive nth roots of unity. If there is a primitive nth root of unity in R then this the standard definition. We then define the standard cyclotomic polynomial cyclotomic n R with coefficients in any ring R.

Main definition

• cyclotomic n R : the n-th cyclotomic polynomial with coefficients in R.

Main results

• Polynomial.degree_cyclotomic : The degree of cyclotomic n is totient n.
• Polynomial.prod_cyclotomic_eq_X_pow_sub_one : X ^ n - 1 = ∏ (cyclotomic i), where i divides n.
• Polynomial.cyclotomic_eq_prod_X_pow_sub_one_pow_moebius : The Möbius inversion formula for cyclotomic n R over an abstract fraction field for R[X].

Implementation details

Our definition of cyclotomic' n R makes sense in any integral domain R, but the interesting results hold if there is a primitive n-th root of unity in R. In particular, our definition is not the standard one unless there is a primitive nth root of unity in R. For example, cyclotomic' 3 ℤ = 1, since there are no primitive cube roots of unity in ℤ. The main example is R = ℂ, we decided to work in general since the difficulties are essentially the same. To get the standard cyclotomic polynomials, we use unique_int_coeff_of_cycl, with R = ℂ, to get a polynomial with integer coefficients and then we map it to R[X], for any ring R.

def Polynomial.cyclotomic' (n : ℕ) (R : Type u_2) [CommRing R] [IsDomain R] : Polynomial R

The modified n-th cyclotomic polynomial with coefficients in R, it is the usual cyclotomic polynomial if there is a primitive n-th root of unity in R.

Equations

• Polynomial.cyclotomic' n R = (primitiveRoots n R).prod fun (μ : R) => Polynomial.X - Polynomial.C μ

@[simp]

theorem Polynomial.cyclotomic'_zero (R : Type u_2) [CommRing R] [IsDomain R] : Polynomial.cyclotomic' 0 R = 1

The zeroth modified cyclotomic polyomial is 1.

@[simp]

theorem Polynomial.cyclotomic'_one (R : Type u_2) [CommRing R] [IsDomain R] : Polynomial.cyclotomic' 1 R = Polynomial.X - 1

The first modified cyclotomic polyomial is X - 1.

@[simp]

theorem Polynomial.cyclotomic'_two (R : Type u_2) [CommRing R] [IsDomain R] (p : ℕ) [CharP R p] (hp : p ≠ 2) : Polynomial.cyclotomic' 2 R = Polynomial.X + 1

The second modified cyclotomic polyomial is X + 1 if the characteristic of R is not 2.

theorem Polynomial.cyclotomic'.monic (n : ℕ) (R : Type u_2) [CommRing R] [IsDomain R] : (Polynomial.cyclotomic' n R).Monic

cyclotomic' n R is monic.

theorem Polynomial.cyclotomic'_ne_zero (n : ℕ) (R : Type u_2) [CommRing R] [IsDomain R] : Polynomial.cyclotomic' n R ≠ 0

cyclotomic' n R is different from 0.

theorem Polynomial.natDegree_cyclotomic' {R : Type u_1} [CommRing R] [IsDomain R] {ζ : R} {n : ℕ} (h : IsPrimitiveRoot ζ n) : (Polynomial.cyclotomic' n R).natDegree = n.totient

The natural degree of cyclotomic' n R is totient n if there is a primitive root of unity in R.

theorem Polynomial.degree_cyclotomic' {R : Type u_1} [CommRing R] [IsDomain R] {ζ : R} {n : ℕ} (h : IsPrimitiveRoot ζ n) : (Polynomial.cyclotomic' n R).degree = ↑n.totient

The degree of cyclotomic' n R is totient n if there is a primitive root of unity in R.

theorem Polynomial.roots_of_cyclotomic (n : ℕ) (R : Type u_2) [CommRing R] [IsDomain R] : (Polynomial.cyclotomic' n R).roots = (primitiveRoots n R).val

The roots of cyclotomic' n R are the primitive n-th roots of unity.

theorem Polynomial.X_pow_sub_one_eq_prod {R : Type u_1} [CommRing R] [IsDomain R] {ζ : R} {n : ℕ} (hpos : 0 < n) (h : IsPrimitiveRoot ζ n) : Polynomial.X ^ n - 1 = (Polynomial.nthRootsFinset n R).prod fun (ζ : R) => Polynomial.X - Polynomial.C ζ

If there is a primitive nth root of unity in K, then X ^ n - 1 = ∏ (X - μ), where μ varies over the n-th roots of unity.

theorem Polynomial.cyclotomic'_splits {K : Type u_1} [Field K] (n : ℕ) : Polynomial.Splits (RingHom.id K) (Polynomial.cyclotomic' n K)

cyclotomic' n K splits.

theorem Polynomial.X_pow_sub_one_splits {K : Type u_1} [Field K] {ζ : K} {n : ℕ} (h : IsPrimitiveRoot ζ n) : Polynomial.Splits (RingHom.id K) (Polynomial.X ^ n - Polynomial.C 1)

If there is a primitive n-th root of unity in K, then X ^ n - 1 splits.

theorem Polynomial.prod_cyclotomic'_eq_X_pow_sub_one {K : Type u_2} [CommRing K] [IsDomain K] {ζ : K} {n : ℕ} (hpos : 0 < n) (h : IsPrimitiveRoot ζ n) : (n.divisors.prod fun (i : ℕ) => Polynomial.cyclotomic' i K) = Polynomial.X ^ n - 1

If there is a primitive n-th root of unity in K, then ∏ i in Nat.divisors n, cyclotomic' i K = X ^ n - 1.

theorem Polynomial.cyclotomic'_eq_X_pow_sub_one_div {K : Type u_2} [CommRing K] [IsDomain K] {ζ : K} {n : ℕ} (hpos : 0 < n) (h : IsPrimitiveRoot ζ n) : Polynomial.cyclotomic' n K = (Polynomial.X ^ n - 1).divByMonic (n.properDivisors.prod fun (i : ℕ) => Polynomial.cyclotomic' i K)

If there is a primitive n-th root of unity in K, then cyclotomic' n K = (X ^ k - 1) /ₘ (∏ i in Nat.properDivisors k, cyclotomic' i K).

theorem Polynomial.int_coeff_of_cyclotomic' {K : Type u_2} [CommRing K] [IsDomain K] {ζ : K} {n : ℕ} (h : IsPrimitiveRoot ζ n) : ∃ (P : Polynomial ℤ), Polynomial.map (Int.castRingHom K) P = Polynomial.cyclotomic' n K ∧ P.degree = (Polynomial.cyclotomic' n K).degree ∧ P.Monic

If there is a primitive n-th root of unity in K, then cyclotomic' n K comes from a monic polynomial with integer coefficients.

theorem Polynomial.unique_int_coeff_of_cycl {K : Type u_2} [CommRing K] [IsDomain K] [CharZero K] {ζ : K} {n : ℕ+} (h : IsPrimitiveRoot ζ ↑n) : ∃! P : Polynomial ℤ, Polynomial.map (Int.castRingHom K) P = Polynomial.cyclotomic' (↑n) K

If K is of characteristic 0 and there is a primitive n-th root of unity in K, then cyclotomic n K comes from a unique polynomial with integer coefficients.

def Polynomial.cyclotomic (n : ℕ) (R : Type u_1) [Ring R] : Polynomial R

The n-th cyclotomic polynomial with coefficients in R.

Equations

• Polynomial.cyclotomic n R = if h : n = 0 then 1 else Polynomial.map (Int.castRingHom R) ⋯.choose

theorem Polynomial.int_cyclotomic_rw {n : ℕ} (h : n ≠ 0) : Polynomial.cyclotomic n ℤ = ⋯.choose

theorem Polynomial.map_cyclotomic_int (n : ℕ) (R : Type u_1) [Ring R] : Polynomial.map (Int.castRingHom R) (Polynomial.cyclotomic n ℤ) = Polynomial.cyclotomic n R

cyclotomic n R comes from cyclotomic n ℤ.

theorem Polynomial.int_cyclotomic_spec (n : ℕ) : Polynomial.map (Int.castRingHom ℂ) (Polynomial.cyclotomic n ℤ) = Polynomial.cyclotomic' n ℂ ∧ (Polynomial.cyclotomic n ℤ).degree = (Polynomial.cyclotomic' n ℂ).degree ∧ (Polynomial.cyclotomic n ℤ).Monic

theorem Polynomial.int_cyclotomic_unique {n : ℕ} {P : Polynomial ℤ} (h : Polynomial.map (Int.castRingHom ℂ) P = Polynomial.cyclotomic' n ℂ) : P = Polynomial.cyclotomic n ℤ

@[simp]

theorem Polynomial.map_cyclotomic (n : ℕ) {R : Type u_1} {S : Type u_2} [Ring R] [Ring S] (f : R →+* S) : Polynomial.map f (Polynomial.cyclotomic n R) = Polynomial.cyclotomic n S

The definition of cyclotomic n R commutes with any ring homomorphism.

theorem Polynomial.cyclotomic.eval_apply {R : Type u_1} {S : Type u_2} (q : R) (n : ℕ) [Ring R] [Ring S] (f : R →+* S) : Polynomial.eval (f q) (Polynomial.cyclotomic n S) = f (Polynomial.eval q (Polynomial.cyclotomic n R))

@[simp]

theorem Polynomial.cyclotomic_zero (R : Type u_1) [Ring R] : Polynomial.cyclotomic 0 R = 1

The zeroth cyclotomic polyomial is 1.

@[simp]

theorem Polynomial.cyclotomic_one (R : Type u_1) [Ring R] : Polynomial.cyclotomic 1 R = Polynomial.X - 1

The first cyclotomic polyomial is X - 1.

theorem Polynomial.cyclotomic.monic (n : ℕ) (R : Type u_1) [Ring R] : (Polynomial.cyclotomic n R).Monic

cyclotomic n is monic.

theorem Polynomial.cyclotomic.isPrimitive (n : ℕ) (R : Type u_1) [CommRing R] : (Polynomial.cyclotomic n R).IsPrimitive

cyclotomic n is primitive.

theorem Polynomial.cyclotomic_ne_zero (n : ℕ) (R : Type u_1) [Ring R] [Nontrivial R] : Polynomial.cyclotomic n R ≠ 0

cyclotomic n R is different from 0.

theorem Polynomial.degree_cyclotomic (n : ℕ) (R : Type u_1) [Ring R] [Nontrivial R] : (Polynomial.cyclotomic n R).degree = ↑n.totient

The degree of cyclotomic n is totient n.

theorem Polynomial.natDegree_cyclotomic (n : ℕ) (R : Type u_1) [Ring R] [Nontrivial R] : (Polynomial.cyclotomic n R).natDegree = n.totient

The natural degree of cyclotomic n is totient n.

theorem Polynomial.degree_cyclotomic_pos (n : ℕ) (R : Type u_1) (hpos : 0 < n) [Ring R] [Nontrivial R] : 0 < (Polynomial.cyclotomic n R).degree

The degree of cyclotomic n R is positive.

theorem Polynomial.prod_cyclotomic_eq_X_pow_sub_one {n : ℕ} (hpos : 0 < n) (R : Type u_1) [CommRing R] : (n.divisors.prod fun (i : ℕ) => Polynomial.cyclotomic i R) = Polynomial.X ^ n - 1

∏ i in Nat.divisors n, cyclotomic i R = X ^ n - 1.

theorem Polynomial.cyclotomic.dvd_X_pow_sub_one (n : ℕ) (R : Type u_1) [Ring R] : Polynomial.cyclotomic n R ∣ Polynomial.X ^ n - 1

theorem Polynomial.prod_cyclotomic_eq_geom_sum {n : ℕ} (h : 0 < n) (R : Type u_1) [CommRing R] : ((n.divisors.erase 1).prod fun (i : ℕ) => Polynomial.cyclotomic i R) = (Finset.range n).sum fun (i : ℕ) => Polynomial.X ^ i

theorem Polynomial.cyclotomic_prime (R : Type u_1) [Ring R] (p : ℕ) [hp : Fact p.Prime] : Polynomial.cyclotomic p R = (Finset.range p).sum fun (i : ℕ) => Polynomial.X ^ i

If p is prime, then cyclotomic p R = ∑ i in range p, X ^ i.

theorem Polynomial.cyclotomic_prime_mul_X_sub_one (R : Type u_1) [Ring R] (p : ℕ) [hn : Fact p.Prime] : Polynomial.cyclotomic p R * (Polynomial.X - 1) = Polynomial.X ^ p - 1

@[simp]

theorem Polynomial.cyclotomic_two (R : Type u_1) [Ring R] : Polynomial.cyclotomic 2 R = Polynomial.X + 1

@[simp]

theorem Polynomial.cyclotomic_three (R : Type u_1) [Ring R] : Polynomial.cyclotomic 3 R = Polynomial.X ^ 2 + Polynomial.X + 1

theorem Polynomial.cyclotomic_dvd_geom_sum_of_dvd (R : Type u_1) [Ring R] {d : ℕ} {n : ℕ} (hdn : d ∣ n) (hd : d ≠ 1) : Polynomial.cyclotomic d R ∣ (Finset.range n).sum fun (i : ℕ) => Polynomial.X ^ i

theorem Polynomial.X_pow_sub_one_mul_prod_cyclotomic_eq_X_pow_sub_one_of_dvd (R : Type u_1) [CommRing R] {d : ℕ} {n : ℕ} (h : d ∈ n.properDivisors) : ((Polynomial.X ^ d - 1) * (n.divisors \ d.divisors).prod fun (x : ℕ) => Polynomial.cyclotomic x R) = Polynomial.X ^ n - 1

theorem Polynomial.X_pow_sub_one_mul_cyclotomic_dvd_X_pow_sub_one_of_dvd (R : Type u_1) [CommRing R] {d : ℕ} {n : ℕ} (h : d ∈ n.properDivisors) : (Polynomial.X ^ d - 1) * Polynomial.cyclotomic n R ∣ Polynomial.X ^ n - 1

theorem Polynomial.cyclotomic_eq_prod_X_pow_sub_one_pow_moebius {n : ℕ} (R : Type u_1) [CommRing R] [IsDomain R] : (algebraMap (Polynomial R) (RatFunc R)) (Polynomial.cyclotomic n R) = n.divisorsAntidiagonal.prod fun (i : ℕ × ℕ) => (algebraMap (Polynomial R) (RatFunc R)) (Polynomial.X ^ i.2 - 1) ^ ArithmeticFunction.moebius i.1

cyclotomic n R can be expressed as a product in a fraction field of R[X] using Möbius inversion.

theorem Polynomial.cyclotomic_eq_X_pow_sub_one_div {R : Type u_1} [CommRing R] {n : ℕ} (hpos : 0 < n) : Polynomial.cyclotomic n R = (Polynomial.X ^ n - 1).divByMonic (n.properDivisors.prod fun (i : ℕ) => Polynomial.cyclotomic i R)

We have cyclotomic n R = (X ^ k - 1) /ₘ (∏ i in Nat.properDivisors k, cyclotomic i K).

theorem Polynomial.X_pow_sub_one_dvd_prod_cyclotomic (R : Type u_1) [CommRing R] {n : ℕ} {m : ℕ} (hpos : 0 < n) (hm : m ∣ n) (hdiff : m ≠ n) : Polynomial.X ^ m - 1 ∣ n.properDivisors.prod fun (i : ℕ) => Polynomial.cyclotomic i R

If m is a proper divisor of n, then X ^ m - 1 divides ∏ i in Nat.properDivisors n, cyclotomic i R.

theorem Polynomial.cyclotomic_eq_prod_X_sub_primitiveRoots {K : Type u_1} [CommRing K] [IsDomain K] {ζ : K} {n : ℕ} (hz : IsPrimitiveRoot ζ n) : Polynomial.cyclotomic n K = (primitiveRoots n K).prod fun (μ : K) => Polynomial.X - Polynomial.C μ

If there is a primitive n-th root of unity in K, then cyclotomic n K = ∏ μ in primitiveRoots n K, (X - C μ). In particular, cyclotomic n K = cyclotomic' n K

theorem Polynomial.eq_cyclotomic_iff {R : Type u_1} [CommRing R] {n : ℕ} (hpos : 0 < n) (P : Polynomial R) : P = Polynomial.cyclotomic n R ↔ (P * n.properDivisors.prod fun (i : ℕ) => Polynomial.cyclotomic i R) = Polynomial.X ^ n - 1

theorem Polynomial.cyclotomic_prime_pow_eq_geom_sum {R : Type u_1} [CommRing R] {p : ℕ} {n : ℕ} (hp : p.Prime) : Polynomial.cyclotomic (p ^ (n + 1)) R = (Finset.range p).sum fun (i : ℕ) => (Polynomial.X ^ p ^ n) ^ i

If p ^ k is a prime power, then cyclotomic (p ^ (n + 1)) R = ∑ i in range p, (X ^ (p ^ n)) ^ i.

theorem Polynomial.cyclotomic_prime_pow_mul_X_pow_sub_one (R : Type u_1) [CommRing R] (p : ℕ) (k : ℕ) [hn : Fact p.Prime] : Polynomial.cyclotomic (p ^ (k + 1)) R * (Polynomial.X ^ p ^ k - 1) = Polynomial.X ^ p ^ (k + 1) - 1

theorem Polynomial.cyclotomic_coeff_zero (R : Type u_1) [CommRing R] {n : ℕ} (hn : 1 < n) : (Polynomial.cyclotomic n R).coeff 0 = 1

The constant term of cyclotomic n R is 1 if 2 ≤ n.

theorem Polynomial.coprime_of_root_cyclotomic {n : ℕ} (hpos : 0 < n) {p : ℕ} [hprime : Fact p.Prime] {a : ℕ} (hroot : (Polynomial.cyclotomic n (ZMod p)).IsRoot ((Nat.castRingHom (ZMod p)) a)) : a.Coprime p

If (a : ℕ) is a root of cyclotomic n (ZMod p), where p is a prime, then a and p are coprime.

theorem Polynomial.orderOf_root_cyclotomic_dvd {n : ℕ} (hpos : 0 < n) {p : ℕ} [Fact p.Prime] {a : ℕ} (hroot : (Polynomial.cyclotomic n (ZMod p)).IsRoot ((Nat.castRingHom (ZMod p)) a)) : orderOf (ZMod.unitOfCoprime a ⋯) ∣ n

If (a : ℕ) is a root of cyclotomic n (ZMod p), then the multiplicative order of a modulo p divides n.