Theorem: ZMod.sum_mul_div_add_sum_mul_div_eq_mul
File: Mathlib/NumberTheory/LegendreSymbol/GaussEisensteinLemmas.lean

Proof steps:
state_before: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
⊢ ∑ a ∈ Ico 1 (p / 2).succ, a * q / p + ∑ a ∈ Ico 1 (q / 2).succ, a * p / q = p / 2 * (q / 2)
state_after: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
hswap :
  (filter (fun x => x.2 * q ≤ x.1 * p) (Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ)).card =
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ)).card
⊢ ∑ a ∈ Ico 1 (p / 2).succ, a * q / p + ∑ a ∈ Ico 1 (q / 2).succ, a * p / q = p / 2 * (q / 2)
tactic: have hswap :
    ((Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ).filter fun x : ℕ × ℕ => x.2 * q ≤ x.1 * p).card =
      ((Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ).filter fun x : ℕ × ℕ =>
        x.1 * q ≤ x.2 * p).card :=
    card_equiv (Equiv.prodComm _ _)
      (fun ⟨_, _⟩ => by
        simp (config := { contextual := true }) only [mem_filter, and_self_iff, Prod.swap_prod_mk,
          forall_true_iff, mem_product, Equiv.prodComm_apply, and_assoc, and_left_comm])
state_before: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
hswap :
  (filter (fun x => x.2 * q ≤ x.1 * p) (Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ)).card =
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ)).card
⊢ ∑ a ∈ Ico 1 (p / 2).succ, a * q / p + ∑ a ∈ Ico 1 (q / 2).succ, a * p / q = p / 2 * (q / 2)
state_after: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
hswap :
  (filter (fun x => x.2 * q ≤ x.1 * p) (Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ)).card =
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ)).card
hdisj :
  Disjoint (filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
⊢ ∑ a ∈ Ico 1 (p / 2).succ, a * q / p + ∑ a ∈ Ico 1 (q / 2).succ, a * p / q = p / 2 * (q / 2)
tactic: have hdisj :
    Disjoint ((Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ).filter fun x : ℕ × ℕ => x.2 * p ≤ x.1 * q)
      ((Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ).filter fun x : ℕ × ℕ => x.1 * q ≤ x.2 * p) := by
    apply disjoint_filter.2 fun x hx hpq hqp => ?_
    have hxp : x.1 < p := lt_of_le_of_lt
      (show x.1 ≤ p / 2 by simp_all only [Nat.lt_succ_iff, mem_Ico, mem_product])
      (Nat.div_lt_self hp.1.pos (by decide))
    have : (x.1 : ZMod p) = 0 := by
      simpa [hq0] using congr_arg ((↑) : ℕ → ZMod p) (le_antisymm hpq hqp)
    apply_fun ZMod.val at this
    rw [val_cast_of_lt hxp, val_zero] at this
    simp only [this, nonpos_iff_eq_zero, mem_Ico, one_ne_zero, false_and_iff, mem_product] at hx
state_before: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
hswap :
  (filter (fun x => x.2 * q ≤ x.1 * p) (Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ)).card =
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ)).card
hdisj :
  Disjoint (filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
⊢ ∑ a ∈ Ico 1 (p / 2).succ, a * q / p + ∑ a ∈ Ico 1 (q / 2).succ, a * p / q = p / 2 * (q / 2)
state_after: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
hswap :
  (filter (fun x => x.2 * q ≤ x.1 * p) (Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ)).card =
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ)).card
hdisj :
  Disjoint (filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
hunion :
  filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ) ∪
      filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ) =
    Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ
⊢ ∑ a ∈ Ico 1 (p / 2).succ, a * q / p + ∑ a ∈ Ico 1 (q / 2).succ, a * p / q = p / 2 * (q / 2)
tactic: have hunion :
      (((Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ).filter fun x : ℕ × ℕ => x.2 * p ≤ x.1 * q) ∪
        (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ).filter fun x : ℕ × ℕ => x.1 * q ≤ x.2 * p) =
      Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ :=
    Finset.ext fun x => by
      have := le_total (x.2 * p) (x.1 * q)
      simp only [mem_union, mem_filter, mem_Ico, mem_product]
      tauto
state_before: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
hswap :
  (filter (fun x => x.2 * q ≤ x.1 * p) (Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ)).card =
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ)).card
hdisj :
  Disjoint (filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
hunion :
  filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ) ∪
      filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ) =
    Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ
⊢ ∑ a ∈ Ico 1 (p / 2).succ, a * q / p + ∑ a ∈ Ico 1 (q / 2).succ, a * p / q = p / 2 * (q / 2)
state_after: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
hswap :
  (filter (fun x => x.2 * q ≤ x.1 * p) (Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ)).card =
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ)).card
hdisj :
  Disjoint (filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
hunion :
  filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ) ∪
      filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ) =
    Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ
⊢ (Ico 1 (p / 2).succ).card * (Ico 1 (q / 2).succ).card = p / 2 * (q / 2)
tactic: rw [sum_Ico_eq_card_lt, sum_Ico_eq_card_lt, hswap, ← card_union_of_disjoint hdisj, hunion,
    card_product]
state_before: p q : ℕ
hp : Fact (Nat.Prime p)
hq0 : ↑q ≠ 0
hswap :
  (filter (fun x => x.2 * q ≤ x.1 * p) (Ico 1 (q / 2).succ ×ˢ Ico 1 (p / 2).succ)).card =
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ)).card
hdisj :
  Disjoint (filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
    (filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ))
hunion :
  filter (fun x => x.2 * p ≤ x.1 * q) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ) ∪
      filter (fun x => x.1 * q ≤ x.2 * p) (Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ) =
    Ico 1 (p / 2).succ ×ˢ Ico 1 (q / 2).succ
⊢ (Ico 1 (p / 2).succ).card * (Ico 1 (q / 2).succ).card = p / 2 * (q / 2)
state_after: no goals
tactic: simp only [card_Ico, tsub_zero, succ_sub_succ_eq_sub]