-- Mathlib/NumberTheory/Multiplicity.lean
theorem odd_sq_dvd_geom_sum₂_sub (hp : Odd p) :
    (p : R) ^ 2 ∣ (∑ i ∈ range p, (a + p * b) ^ i * a ^ (p - 1 - i)) - p * a ^ (p - 1)

Proof state:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
this : ∑ x ∈ range p, ↑x = ↑(∑ x ∈ range p, x)
⊢ (Ideal.Quotient.mk (span {s})) ↑p ^ 2 * (Ideal.Quotient.mk (span {s})) b *
        (Ideal.Quotient.mk (span {s})) (a ^ (p - 2)) *
      (Ideal.Quotient.mk (span {s})) ↑((p - 1) / 2) =
    0

Tactic:
rw [mul_assoc, mul_assoc]

Result:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
this : ∑ x ∈ range p, ↑x = ↑(∑ x ∈ range p, x)
⊢ (Ideal.Quotient.mk (span {s})) ↑p ^ 2 *
      ((Ideal.Quotient.mk (span {s})) b *
        ((Ideal.Quotient.mk (span {s})) (a ^ (p - 2)) * (Ideal.Quotient.mk (span {s})) ↑((p - 1) / 2))) =
    0

Proof state:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
⊢ (Ideal.Quotient.mk (span {s})) (∑ x ∈ range p, a ^ (x - 1 + (p - 1 - x)) * ↑p * b * ↑x) +
      (Ideal.Quotient.mk (span {s})) (∑ x ∈ range p, a ^ (x + (p - 1 - x))) =
    (Ideal.Quotient.mk (span {s})) (∑ x ∈ range p, a ^ (x - 1) * (a ^ (p - 1 - x) * (↑p * (b * ↑x)))) +
      (Ideal.Quotient.mk (span {s})) (∑ x ∈ range p, a ^ (x + (p - 1 - x)))

Tactic:
congr

Result:
case e_a.h.e_6.h.e_f
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
⊢ (fun x => a ^ (x - 1 + (p - 1 - x)) * ↑p * b * ↑x) = fun x => a ^ (x - 1) * (a ^ (p - 1 - x) * (↑p * (b * ↑x)))

Proof state:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
this : ∑ x ∈ range p, ↑x = ↑(∑ x ∈ range p, x)
⊢ (Ideal.Quotient.mk (span {s})) ↑p * (Ideal.Quotient.mk (span {s})) b *
      ((Ideal.Quotient.mk (span {s})) (a ^ (p - 2)) *
        ((Ideal.Quotient.mk (span {s})) ↑p * (Ideal.Quotient.mk (span {s})) ↑((p - 1) / 2))) =
    0

Tactic:
ring_nf

Result:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
this : ∑ x ∈ range p, ↑x = ↑(∑ x ∈ range p, x)
⊢ (Ideal.Quotient.mk (span {s})) ↑p ^ 2 * (Ideal.Quotient.mk (span {s})) b *
        (Ideal.Quotient.mk (span {s})) (a ^ (p - 2)) *
      (Ideal.Quotient.mk (span {s})) ↑((p - 1) / 2) =
    0

Proof state:
case succ
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
x : ℕ
hx : x + 1 ∈ range p
⊢ a ^ (x + 1 - 1 + (p - 1 - (x + 1))) * ↑p * b * ↑(x + 1) = ↑p * b * a ^ (p - 2) * ↑(x + 1)

Tactic:
have : x.succ - 1 + (p - 1 - x.succ) = p - 2 := by
  rw [← Nat.add_sub_assoc (Nat.le_sub_one_of_lt (Finset.mem_range.mp hx))]
  exact congr_arg Nat.pred (Nat.add_sub_cancel_left _ _)

Result:
case succ
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
x : ℕ
hx : x + 1 ∈ range p
this : x.succ - 1 + (p - 1 - x.succ) = p - 2
⊢ a ^ (x + 1 - 1 + (p - 1 - (x + 1))) * ↑p * b * ↑(x + 1) = ↑p * b * a ^ (p - 2) * ↑(x + 1)

Proof state:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
⊢ (Ideal.Quotient.mk (span {s})) (∑ x ∈ range p, a ^ (x - 1) * (a ^ (p - 1 - x) * (↑p * (b * ↑x)))) +
      (Ideal.Quotient.mk (span {s})) (∑ _x ∈ range p, a ^ (p - 1)) =
    (Ideal.Quotient.mk (span {s})) (∑ x ∈ range p, a ^ (x - 1) * (a ^ (p - 1 - x) * (↑p * (b * ↑x)))) +
      (Ideal.Quotient.mk (span {s})) (↑p * a ^ (p - 1))

Tactic:
simp only [add_right_inj, Finset.sum_const, Finset.card_range, nsmul_eq_mul]

Result:
no goals

Proof state:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
⊢ (Ideal.Quotient.mk (span {↑p ^ 2})) (∑ i ∈ range p, (a + ↑p * b) ^ i * a ^ (p - 1 - i)) =
    (Ideal.Quotient.mk (span {↑p ^ 2})) (↑p * a ^ (p - 1))

Tactic:
let s : R := (p : R)^2

Result:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
⊢ (Ideal.Quotient.mk (span {↑p ^ 2})) (∑ i ∈ range p, (a + ↑p * b) ^ i * a ^ (p - 1 - i)) =
    (Ideal.Quotient.mk (span {↑p ^ 2})) (↑p * a ^ (p - 1))

Proof state:
case succ
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
x : ℕ
hx : x + 1 ∈ range p
this : x.succ - 1 + (p - 1 - x.succ) = p - 2
⊢ a ^ (p - 2) * ↑p * b * ↑(x + 1) = ↑p * b * a ^ (p - 2) * ↑(x + 1)

Tactic:
ring1

Result:
no goals

Proof state:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
⊢ (Ideal.Quotient.mk (span {s})) (∑ i ∈ range p, (a + ↑p * b) ^ i * a ^ (p - 1 - i)) =
    ∑ i ∈ range p, (Ideal.Quotient.mk (span {s})) ((a ^ (i - 1) * (↑p * b) * ↑i + a ^ i) * a ^ (p - 1 - i))

Tactic:
simp_rw [s, RingHom.map_geom_sum₂, ← map_pow, h1, ← _root_.map_mul]

Result:
no goals

Proof state:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
this : ∑ x ∈ range p, ↑x = ↑(∑ x ∈ range p, x)
⊢ (Ideal.Quotient.mk (span {s})) ↑p ^ 2 *
      ((Ideal.Quotient.mk (span {s})) b *
        ((Ideal.Quotient.mk (span {s})) (a ^ (p - 2)) * (Ideal.Quotient.mk (span {s})) ↑((p - 1) / 2))) =
    0

Tactic:
refine mul_eq_zero_of_left ?_ _

Result:
R : Type u_1
inst✝ : CommRing R
a b : R
p : ℕ
hp : Odd p
h1 :
  ∀ (i : ℕ),
    (Ideal.Quotient.mk (span {↑p ^ 2})) ((a + ↑p * b) ^ i) =
      (Ideal.Quotient.mk (span {↑p ^ 2})) (a ^ (i - 1) * (↑p * b) * ↑i + a ^ i)
s : R := ↑p ^ 2
this : ∑ x ∈ range p, ↑x = ↑(∑ x ∈ range p, x)
⊢ (Ideal.Quotient.mk (span {s})) ↑p ^ 2 = 0