-- Mathlib/Topology/Algebra/Order/Field.lean
theorem IsTopologicalRing.of_norm {R 𝕜 : Type*} [NonUnitalNonAssocRing R] [LinearOrderedField 𝕜]
    [TopologicalSpace R] [IsTopologicalAddGroup R] (norm : R → 𝕜)
    (norm_nonneg : ∀ x, 0 ≤ norm x) (norm_mul_le : ∀ x y, norm (x * y) ≤ norm x * norm y)
    (nhds_basis : (𝓝 (0 : R)).HasBasis ((0 : 𝕜) < ·) (fun ε ↦ { x | norm x < ε })) :
    IsTopologicalRing R

Proof state:
case hmul
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
⊢ Tendsto (uncurry fun x1 x2 => x1 * x2) (𝓝 0 ×ˢ 𝓝 0) (𝓝 0)

case hmul_left
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
⊢ ∀ (x₀ : R), Tendsto (fun x => x₀ * x) (𝓝 0) (𝓝 0)

case hmul_right
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
⊢ ∀ (x₀ : R), Tendsto (fun x => x * x₀) (𝓝 0) (𝓝 0)

Tactic:
case hmul =>
  refine ((nhds_basis.prod nhds_basis).tendsto_iff nhds_basis).2 fun ε ε0 ↦ ?_
  refine ⟨(1, ε), ⟨one_pos, ε0⟩, fun (x, y) ⟨hx, hy⟩ => ?_⟩
  simp only [sub_zero] at *
  calc norm (x * y) ≤ norm x * norm y := norm_mul_le _ _
  _ < ε := (mul_le_of_le_one_left (norm_nonneg _) hx.le).trans_lt hy

Result:
case hmul_left
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
⊢ ∀ (x₀ : R), Tendsto (fun x => x₀ * x) (𝓝 0) (𝓝 0)

case hmul_right
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
⊢ ∀ (x₀ : R), Tendsto (fun x => x * x₀) (𝓝 0) (𝓝 0)

Proof state:
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
⊢ IsTopologicalRing R

Tactic:
have h0 : ∀ f : R → R, ∀ c ≥ (0 : 𝕜), (∀ x, norm (f x) ≤ c * norm x) →
    Tendsto f (𝓝 0) (𝓝 0) := by
  refine fun f c c0 hf ↦ (nhds_basis.tendsto_iff nhds_basis).2 fun ε ε0 ↦ ?_
  rcases exists_pos_mul_lt ε0 c with ⟨δ, δ0, hδ⟩
  refine ⟨δ, δ0, fun x hx ↦ (hf _).trans_lt ?_⟩
  exact (mul_le_mul_of_nonneg_left (le_of_lt hx) c0).trans_lt hδ

Result:
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
⊢ IsTopologicalRing R

Proof state:
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
ε : 𝕜
ε0 : 0 < ε
x✝¹ : R × R
x y : R
x✝ : (x, y) ∈ {x | norm x < (1, ε).1} ×ˢ {x | norm x < (1, ε).2}
hx : (x, y).1 ∈ {x | norm x < (1, ε).1}
hy : (x, y).2 ∈ {x | norm x < (1, ε).2}
⊢ uncurry (fun x1 x2 => x1 * x2) (x, y) ∈ {x | norm x < ε}

Tactic:
simp only [sub_zero] at *

Result:
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
ε : 𝕜
ε0 : 0 < ε
x✝¹ : R × R
x y : R
x✝ : (x, y) ∈ {x | norm x < 1} ×ˢ {x | norm x < ε}
hx : x ∈ {x | norm x < 1}
hy : y ∈ {x | norm x < ε}
⊢ uncurry (fun x1 x2 => x1 * x2) (x, y) ∈ {x | norm x < ε}

Proof state:
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
⊢ ∀ (x₀ : R), Tendsto (fun x => x₀ * x) (𝓝 0) (𝓝 0)

Tactic:
exact fun x => h0 _ (norm x) (norm_nonneg _) (norm_mul_le x)

Result:
no goals

Proof state:
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
ε : 𝕜
ε0 : 0 < ε
⊢ ∃ ia,
    (0 < ia.1 ∧ 0 < ia.2) ∧
      ∀ x ∈ {x | norm x < ia.1} ×ˢ {x | norm x < ia.2}, uncurry (fun x1 x2 => x1 * x2) x ∈ {x | norm x < ε}

Tactic:
refine ⟨(1, ε), ⟨one_pos, ε0⟩, fun (x, y) ⟨hx, hy⟩ => ?_⟩

Result:
R : Type u_1
𝕜 : Type u_2
inst✝³ : NonUnitalNonAssocRing R
inst✝² : LinearOrderedField 𝕜
inst✝¹ : TopologicalSpace R
inst✝ : IsTopologicalAddGroup R
norm : R → 𝕜
norm_nonneg : ∀ (x : R), 0 ≤ norm x
norm_mul_le : ∀ (x y : R), norm (x * y) ≤ norm x * norm y
nhds_basis : (𝓝 0).HasBasis (fun x => 0 < x) fun ε => {x | norm x < ε}
h0 : ∀ (f : R → R), ∀ c ≥ 0, (∀ (x : R), norm (f x) ≤ c * norm x) → Tendsto f (𝓝 0) (𝓝 0)
ε : 𝕜
ε0 : 0 < ε
x✝¹ : R × R
x y : R
x✝ : (x, y) ∈ {x | norm x < (1, ε).1} ×ˢ {x | norm x < (1, ε).2}
hx : (x, y).1 ∈ {x | norm x < (1, ε).1}
hy : (x, y).2 ∈ {x | norm x < (1, ε).2}
⊢ uncurry (fun x1 x2 => x1 * x2) (x, y) ∈ {x | norm x < ε}