-- Mathlib/NumberTheory/Padics/RingHoms.lean
theorem lift_sub_val_mem_span (r : R) (n : ℕ) :
    lift f_compat r - (f n r).val ∈ (Ideal.span {(p : ℤ_[p]) ^ n})

Proof state:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ ↑(nthHom f r (n ⊔ k)) - ↑((f n) r).val + (limNthHom f_compat r - ↑(nthHom f r (n ⊔ k))) ∈ Ideal.span {↑p ^ n}

Tactic:
apply Ideal.add_mem _ _ this

Result:
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ ↑(nthHom f r (n ⊔ k)) - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}

Proof state:
case h.e'_3
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ ↑p ^ n = (Int.castRingHom ℤ_[p]) (↑p ^ n)

Tactic:
rw [map_pow]

Result:
case h.e'_3
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ ↑p ^ n = (Int.castRingHom ℤ_[p]) ↑p ^ n

Proof state:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ limNthHom f_compat r - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}

Tactic:
rw [sub_eq_sub_add_sub (limNthHom f_compat r) _ ↑(nthHom f r (max n k))]

Result:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ ↑(nthHom f r (n ⊔ k)) - ↑((f n) r).val + (limNthHom f_compat r - ↑(nthHom f r (n ⊔ k))) ∈ Ideal.span {↑p ^ n}

Proof state:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
⊢ (lift f_compat) r - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}

Tactic:
have := le_of_lt (hk (max n k) (le_max_right _ _))

Result:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : ‖limNthHom f_compat r - ↑(nthHom f r (n ⊔ k))‖ ≤ ↑p ^ (-↑n)
⊢ (lift f_compat) r - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}

Proof state:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ (lift f_compat) r - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}

Tactic:
dsimp [lift]

Result:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ limNthHom f_compat r - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}

Proof state:
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ ↑(nthHom f r (n ⊔ k)) - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}

Tactic:
rw [Ideal.mem_span_singleton]

Result:
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ ↑p ^ n ∣ ↑(nthHom f r (n ⊔ k)) - ↑((f n) r).val

Proof state:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : ‖limNthHom f_compat r - ↑(nthHom f r (n ⊔ k))‖ ≤ ↑p ^ (-↑n)
⊢ (lift f_compat) r - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}

Tactic:
rw [norm_le_pow_iff_mem_span_pow] at this

Result:
case intro
R : Type u_1
inst✝ : NonAssocSemiring R
p : ℕ
f : (k : ℕ) → R →+* ZMod (p ^ k)
hp_prime : Fact (Nat.Prime p)
f_compat : ∀ (k1 k2 : ℕ) (hk : k1 ≤ k2), (ZMod.castHom ⋯ (ZMod (p ^ k1))).comp (f k2) = f k1
r : R
n k : ℕ
hk : ∀ n_1 ≥ k, ‖limNthHom f_compat r - ↑(nthHom f r n_1)‖ < ↑p ^ (-↑n)
this : limNthHom f_compat r - ↑(nthHom f r (n ⊔ k)) ∈ Ideal.span {↑p ^ n}
⊢ (lift f_compat) r - ↑((f n) r).val ∈ Ideal.span {↑p ^ n}