Theorem: Polynomial.cyclotomic_coeff_zero
File: Mathlib/RingTheory/Polynomial/Cyclotomic/Basic.lean

Proof steps:
state_before: R : Type u_1
inst✝ : CommRing R
n : ℕ
hn : 1 < n
⊢ (cyclotomic n R).coeff 0 = 1
state_after: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
⊢ (cyclotomic n R).coeff 0 = 1
tactic: induction' n using Nat.strong_induction_on with n hi
state_before: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
⊢ (cyclotomic n R).coeff 0 = 1
state_after: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
hprod : ∏ i ∈ n.properDivisors, (cyclotomic i R).coeff 0 = -1
⊢ (cyclotomic n R).coeff 0 = 1
tactic: have hprod : (∏ i ∈ Nat.properDivisors n, (Polynomial.cyclotomic i R).coeff 0) = -1 := by
    rw [← Finset.insert_erase (Nat.one_mem_properDivisors_iff_one_lt.2
      (lt_of_lt_of_le one_lt_two hn)), Finset.prod_insert (Finset.not_mem_erase 1 _),
      cyclotomic_one R]
    have hleq : ∀ j ∈ n.properDivisors.erase 1, 2 ≤ j := by
      intro j hj
      apply Nat.succ_le_of_lt
      exact (Ne.le_iff_lt (Finset.mem_erase.1 hj).1.symm).mp
        (Nat.succ_le_of_lt (Nat.pos_of_mem_properDivisors (Finset.mem_erase.1 hj).2))
    have hcongr : ∀ j ∈ n.properDivisors.erase 1, (cyclotomic j R).coeff 0 = 1 := by
      intro j hj
      exact hi j (Nat.mem_properDivisors.1 (Finset.mem_erase.1 hj).2).2 (hleq j hj)
    have hrw : (∏ x ∈ n.properDivisors.erase 1, (cyclotomic x R).coeff 0) = 1 := by
      rw [Finset.prod_congr (refl (n.properDivisors.erase 1)) hcongr]
      simp only [Finset.prod_const_one]
    simp only [hrw, mul_one, zero_sub, coeff_one_zero, coeff_X_zero, coeff_sub]
state_before: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
hprod : ∏ i ∈ n.properDivisors, (cyclotomic i R).coeff 0 = -1
⊢ (cyclotomic n R).coeff 0 = 1
state_after: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
hprod : ∏ i ∈ n.properDivisors, (cyclotomic i R).coeff 0 = -1
heq : (X ^ n - 1).coeff 0 = -(cyclotomic n R).coeff 0
⊢ (cyclotomic n R).coeff 0 = 1
tactic: have heq : (X ^ n - 1 : R[X]).coeff 0 = -(cyclotomic n R).coeff 0 := by
    rw [← prod_cyclotomic_eq_X_pow_sub_one (zero_le_one.trans_lt hn), ←
      Nat.cons_self_properDivisors hn.ne_bot, Finset.prod_cons, mul_coeff_zero, coeff_zero_prod,
      hprod, mul_neg, mul_one]
state_before: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
hprod : ∏ i ∈ n.properDivisors, (cyclotomic i R).coeff 0 = -1
heq : (X ^ n - 1).coeff 0 = -(cyclotomic n R).coeff 0
⊢ (cyclotomic n R).coeff 0 = 1
state_after: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
hprod : ∏ i ∈ n.properDivisors, (cyclotomic i R).coeff 0 = -1
heq : (X ^ n - 1).coeff 0 = -(cyclotomic n R).coeff 0
hzero : (X ^ n - 1).coeff 0 = -1
⊢ (cyclotomic n R).coeff 0 = 1
tactic: have hzero : (X ^ n - 1 : R[X]).coeff 0 = (-1 : R) := by
    rw [coeff_zero_eq_eval_zero _]
    simp only [zero_pow (by positivity : n ≠ 0), eval_X, eval_one, zero_sub, eval_pow, eval_sub]
state_before: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
hprod : ∏ i ∈ n.properDivisors, (cyclotomic i R).coeff 0 = -1
heq : (X ^ n - 1).coeff 0 = -(cyclotomic n R).coeff 0
hzero : (X ^ n - 1).coeff 0 = -1
⊢ (cyclotomic n R).coeff 0 = 1
state_after: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
hprod : ∏ i ∈ n.properDivisors, (cyclotomic i R).coeff 0 = -1
heq : -1 = -(cyclotomic n R).coeff 0
hzero : (X ^ n - 1).coeff 0 = -1
⊢ (cyclotomic n R).coeff 0 = 1
tactic: rw [hzero] at heq
state_before: case h
R : Type u_1
inst✝ : CommRing R
n : ℕ
hi : ∀ m < n, 1 < m → (cyclotomic m R).coeff 0 = 1
hn : 1 < n
hprod : ∏ i ∈ n.properDivisors, (cyclotomic i R).coeff 0 = -1
heq : -1 = -(cyclotomic n R).coeff 0
hzero : (X ^ n - 1).coeff 0 = -1
⊢ (cyclotomic n R).coeff 0 = 1
state_after: no goals
tactic: exact neg_inj.mp (Eq.symm heq)