-- Mathlib/Algebra/Polynomial/UnitTrinomial.lean
theorem irreducible_aux1 {k m n : ℕ} (hkm : k < m) (hmn : m < n) (u v w : Units ℤ)
    (hp : p = trinomial k m n (u : ℤ) v w) :
    C (v : ℤ) * (C (u : ℤ) * X ^ (m + n) + C (w : ℤ) * X ^ (n - m + k + n)) =
      ⟨Finsupp.filter (· ∈ Set.Ioo (k + n) (n + n)) (p * p.mirror).toFinsupp⟩

Proof state:
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ C ↑v * (monomial (m + n)) ↑u + C ↑v * (monomial (n - m + k + n)) ↑w =
    {
      toFinsupp :=
        Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (k + k) (↑u * ↑w)) +
                Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (k + (n - m + k)) (↑u * ↑v)) +
              Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (k + n) (↑u * ↑u)) +
            (Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (m + k) (↑v * ↑w)) +
                Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (m + (n - m + k)) (↑v * ↑v)) +
              Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (m + n) (↑v * ↑u))) +
          (Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (n + k) (↑w * ↑w)) +
              Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (n + (n - m + k)) (↑w * ↑v)) +
            Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (Finsupp.single (n + n) (↑w * ↑u))) }

Tactic:
rw [Finsupp.filter_single_of_neg, Finsupp.filter_single_of_neg, Finsupp.filter_single_of_neg,
  Finsupp.filter_single_of_neg, Finsupp.filter_single_of_neg, Finsupp.filter_single_of_pos,
  Finsupp.filter_single_of_neg, Finsupp.filter_single_of_pos, Finsupp.filter_single_of_neg]

Result:
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ C ↑v * (monomial (m + n)) ↑u + C ↑v * (monomial (n - m + k + n)) ↑w =
    {
      toFinsupp :=
        0 + 0 + 0 + (0 + 0 + Finsupp.single (m + n) (↑v * ↑u)) + (0 + Finsupp.single (n + (n - m + k)) (↑w * ↑v) + 0) }

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ n + n ∉ Set.Ioo (k + n) (n + n)

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ n + (n - m + k) ∈ Set.Ioo (k + n) (n + n)

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ n + k ∉ Set.Ioo (k + n) (n + n)

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ m + n ∈ Set.Ioo (k + n) (n + n)

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ m + (n - m + k) ∉ Set.Ioo (k + n) (n + n)

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ m + k ∉ Set.Ioo (k + n) (n + n)

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ k + n ∉ Set.Ioo (k + n) (n + n)

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ k + (n - m + k) ∉ Set.Ioo (k + n) (n + n)

case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ k + k ∉ Set.Ioo (k + n) (n + n)

Proof state:
case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ m + (n - m + k) ∉ Set.Ioo (k + n) (n + n)

Tactic:
rw [← add_assoc, add_tsub_cancel_of_le hmn.le, add_comm]

Result:
case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ n + k ∉ Set.Ioo (n + k) (n + n)

Proof state:
case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
h : m + k ∈ Set.Ioo (k + n) (n + n)
this : k + n < m + k
⊢ False

Tactic:
rw [add_comm, add_lt_add_iff_right] at this

Result:
case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
h : m + k ∈ Set.Ioo (k + n) (n + n)
this : n < m
⊢ False

Proof state:
case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ k + n < n + (n - m + k)

Tactic:
rwa [add_comm, add_lt_add_iff_left, lt_add_iff_pos_left, tsub_pos_iff_lt]

Result:
no goals

Proof state:
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ C ↑v * (C ↑u * X ^ (m + n) + C ↑w * X ^ (n - m + k + n)) =
    { toFinsupp := Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n)) (p * p.mirror).toFinsupp }

Tactic:
rw [hp, trinomial_mirror hkm hmn u.ne_zero w.ne_zero]

Result:
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ C ↑v * (C ↑u * X ^ (m + n) + C ↑w * X ^ (n - m + k + n)) =
    {
      toFinsupp :=
        Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n))
          (trinomial k m n ↑u ↑v ↑w * trinomial k (n - m + k) n ↑w ↑v ↑u).toFinsupp }

Proof state:
case h
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ k + (n - m + k) ∉ Set.Ioo (k + n) (n + n)

Tactic:
exact fun h => asymm ((add_lt_add_iff_left k).mp h.1) key

Result:
no goals

Proof state:
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
⊢ n - m + k < n

Tactic:
rwa [← lt_tsub_iff_right, tsub_lt_tsub_iff_left_of_le hmn.le]

Result:
no goals

Proof state:
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ C ↑v * (C ↑u * X ^ (m + n) + C ↑w * X ^ (n - m + k + n)) =
    {
      toFinsupp :=
        Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n))
          (trinomial k m n ↑u ↑v ↑w * trinomial k (n - m + k) n ↑w ↑v ↑u).toFinsupp }

Tactic:
simp_rw [trinomial_def, C_mul_X_pow_eq_monomial, add_mul, mul_add, monomial_mul_monomial,
  toFinsupp_add, toFinsupp_monomial]

Result:
p : ℤ[X]
k m n : ℕ
hkm : k < m
hmn : m < n
u v w : ℤˣ
hp : p = trinomial k m n ↑u ↑v ↑w
key : n - m + k < n
⊢ C ↑v * (monomial (m + n)) ↑u + C ↑v * (monomial (n - m + k + n)) ↑w =
    {
      toFinsupp :=
        Finsupp.filter (fun x => x ∈ Set.Ioo (k + n) (n + n))
          (Finsupp.single (k + k) (↑u * ↑w) + Finsupp.single (k + (n - m + k)) (↑u * ↑v) +
                Finsupp.single (k + n) (↑u * ↑u) +
              (Finsupp.single (m + k) (↑v * ↑w) + Finsupp.single (m + (n - m + k)) (↑v * ↑v) +
                Finsupp.single (m + n) (↑v * ↑u)) +
            (Finsupp.single (n + k) (↑w * ↑w) + Finsupp.single (n + (n - m + k)) (↑w * ↑v) +
              Finsupp.single (n + n) (↑w * ↑u))) }