Vieta's Formula

The main result is Multiset.prod_X_add_C_eq_sum_esymm, which shows that the product of linear terms X + λ with λ in a Multiset s is equal to a linear combination of the symmetric functions esymm s.

From this, we deduce MvPolynomial.prod_X_add_C_eq_sum_esymm which is the equivalent formula for the product of linear terms X + X i with i in a Fintype σ as a linear combination of the symmetric polynomials esymm σ R j.

For R be an integral domain (so that p.roots is defined for any p : R[X] as a multiset), we derive Polynomial.coeff_eq_esymm_roots_of_card, the relationship between the coefficients and the roots of p for a polynomial p that splits (i.e. having as many roots as its degree).

theorem Multiset.prod_X_add_C_eq_sum_esymm {R : Type u_1} [CommSemiring R] (s : Multiset R) : Multiset.prod (Multiset.map (fun r => Polynomial.X + ↑Polynomial.C r) s) = Finset.sum (Finset.range (↑Multiset.card s + 1)) fun j => ↑Polynomial.C (Multiset.esymm s j) * Polynomial.X ^ (↑Multiset.card s - j)

A sum version of Vieta's formula for Multiset: the product of the linear terms X + λ where λ runs through a multiset s is equal to a linear combination of the symmetric functions esymm s of the λ's .

theorem Multiset.prod_X_add_C_coeff {R : Type u_1} [CommSemiring R] (s : Multiset R) {k : ℕ} (h : k ≤ ↑Multiset.card s) : Polynomial.coeff (Multiset.prod (Multiset.map (fun r => Polynomial.X + ↑Polynomial.C r) s)) k = Multiset.esymm s (↑Multiset.card s - k)

Vieta's formula for the coefficients of the product of linear terms X + λ where λ runs through a multiset s : the kth coefficient is the symmetric function esymm (card s - k) s.

theorem Multiset.prod_X_add_C_coeff' {R : Type u_1} [CommSemiring R] {σ : Type u_2} (s : Multiset σ) (r : σ → R) {k : ℕ} (h : k ≤ ↑Multiset.card s) : Polynomial.coeff (Multiset.prod (Multiset.map (fun i => Polynomial.X + ↑Polynomial.C (r i)) s)) k = Multiset.esymm (Multiset.map r s) (↑Multiset.card s - k)

theorem Finset.prod_X_add_C_coeff {R : Type u_1} [CommSemiring R] {σ : Type u_2} (s : Finset σ) (r : σ → R) {k : ℕ} (h : k ≤ Finset.card s) : Polynomial.coeff (Finset.prod s fun i => Polynomial.X + ↑Polynomial.C (r i)) k = Finset.sum (Finset.powersetLen (Finset.card s - k) s) fun t => Finset.prod t fun i => r i

theorem Multiset.esymm_neg {R : Type u_1} [CommRing R] (s : Multiset R) (k : ℕ) : Multiset.esymm (Multiset.map Neg.neg s) k = (-1) ^ k * Multiset.esymm s k

theorem Multiset.prod_X_sub_X_eq_sum_esymm {R : Type u_1} [CommRing R] (s : Multiset R) : Multiset.prod (Multiset.map (fun t => Polynomial.X - ↑Polynomial.C t) s) = Finset.sum (Finset.range (↑Multiset.card s + 1)) fun j => (-1) ^ j * (↑Polynomial.C (Multiset.esymm s j) * Polynomial.X ^ (↑Multiset.card s - j))

theorem Multiset.prod_X_sub_C_coeff {R : Type u_1} [CommRing R] (s : Multiset R) {k : ℕ} (h : k ≤ ↑Multiset.card s) : Polynomial.coeff (Multiset.prod (Multiset.map (fun t => Polynomial.X - ↑Polynomial.C t) s)) k = (-1) ^ (↑Multiset.card s - k) * Multiset.esymm s (↑Multiset.card s - k)

theorem Polynomial.coeff_eq_esymm_roots_of_card {R : Type u_1} [CommRing R] [IsDomain R] {p : Polynomial R} (hroots : ↑Multiset.card (Polynomial.roots p) = Polynomial.natDegree p) {k : ℕ} (h : k ≤ Polynomial.natDegree p) : Polynomial.coeff p k = Polynomial.leadingCoeff p * (-1) ^ (Polynomial.natDegree p - k) * Multiset.esymm (Polynomial.roots p) (Polynomial.natDegree p - k)

Vieta's formula for the coefficients and the roots of a polynomial over an integral domain with as many roots as its degree.

theorem Polynomial.coeff_eq_esymm_roots_of_splits {F : Type u_2} [Field F] {p : Polynomial F} (hsplit : Polynomial.Splits (RingHom.id F) p) {k : ℕ} (h : k ≤ Polynomial.natDegree p) : Polynomial.coeff p k = Polynomial.leadingCoeff p * (-1) ^ (Polynomial.natDegree p - k) * Multiset.esymm (Polynomial.roots p) (Polynomial.natDegree p - k)

Vieta's formula for split polynomials over a field.

theorem MvPolynomial.prod_C_add_X_eq_sum_esymm (R : Type u_1) (σ : Type u_2) [CommSemiring R] [Fintype σ] : (Finset.prod Finset.univ fun i => Polynomial.X + ↑Polynomial.C (MvPolynomial.X i)) = Finset.sum (Finset.range (Fintype.card σ + 1)) fun j => ↑Polynomial.C (MvPolynomial.esymm σ R j) * Polynomial.X ^ (Fintype.card σ - j)

A sum version of Vieta's formula for MvPolynomial: viewing X i as variables, the product of linear terms λ + X i is equal to a linear combination of the symmetric polynomials esymm σ R j.

theorem MvPolynomial.prod_X_add_C_coeff (R : Type u_1) (σ : Type u_2) [CommSemiring R] [Fintype σ] (k : ℕ) (h : k ≤ Fintype.card σ) : Polynomial.coeff (Finset.prod Finset.univ fun i => Polynomial.X + ↑Polynomial.C (MvPolynomial.X i)) k = MvPolynomial.esymm σ R (Fintype.card σ - k)