-- Mathlib/Analysis/SpecificLimits/Normed.lean
theorem Monotone.cauchySeq_series_mul_of_tendsto_zero_of_bounded (hfa : Monotone f)
    (hf0 : Tendsto f atTop (𝓝 0)) (hgb : ∀ n, ‖∑ i ∈ range n, z i‖ ≤ b) :
    CauchySeq fun n ↦ ∑ i ∈ range n, f i • z i

Proof state:
case refine_2
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
n : ℕ
⊢ ‖(Finset.range (n + 1)).sum z‖ * ‖f (n + 1) + -f n‖ ≤ b * |f (n + 1) - f n|

Tactic:
exact mul_le_mul_of_nonneg_right (hgb _) (abs_nonneg _)

Result:
no goals

Proof state:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ i ∈ Finset.range n, f i • z i

Tactic:
rw [← cauchySeq_shift 1]

Result:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ i ∈ Finset.range (n + 1), f i • z i

Proof state:
case refine_1
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => b * ∑ i ∈ Finset.range n, (f i.succ - f i)

Tactic:
apply Real.uniformContinuous_const_mul.comp_cauchySeq

Result:
case refine_1
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ i ∈ Finset.range n, (f i.succ - f i)

Proof state:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n =>
    f n • ∑ i ∈ Finset.range n.succ, z i + -∑ x ∈ Finset.range n, (f (x + 1) + -f x) • ∑ i ∈ Finset.range (x + 1), z i

Tactic:
apply (NormedField.tendsto_zero_smul_of_tendsto_zero_of_bounded hf0
  ⟨b, eventually_map.mpr <| Eventually.of_forall fun n ↦ hgb <| n + 1⟩).cauchySeq.add

Result:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => -∑ x ∈ Finset.range n, (f (x + 1) + -f x) • ∑ i ∈ Finset.range (x + 1), z i

Proof state:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ x ∈ Finset.range n, (f (x + 1) + -f x) • ∑ i ∈ Finset.range (x + 1), z i

Tactic:
refine cauchySeq_range_of_norm_bounded _ ?_
  (fun n ↦ ?_ : ∀ n, ‖(f (n + 1) + -f n) • (Finset.range (n + 1)).sum z‖ ≤ b * |f (n + 1) - f n|)

Result:
case refine_1
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ i ∈ Finset.range n, b * |f (i + 1) - f i|

case refine_2
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
n : ℕ
⊢ ‖(f (n + 1) + -f n) • (Finset.range (n + 1)).sum z‖ ≤ b * |f (n + 1) - f n|

Proof state:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => -∑ x ∈ Finset.range n, (f (x + 1) + -f x) • ∑ i ∈ Finset.range (x + 1), z i

Tactic:
refine CauchySeq.neg ?_

Result:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ x ∈ Finset.range n, (f (x + 1) + -f x) • ∑ i ∈ Finset.range (x + 1), z i

Proof state:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ i ∈ Finset.range (n + 1), f i • z i

Tactic:
simp_rw [Finset.sum_range_by_parts _ _ (Nat.succ _), sub_eq_add_neg, Nat.succ_sub_succ_eq_sub,
  tsub_zero]

Result:
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n =>
    f n • ∑ i ∈ Finset.range n.succ, z i + -∑ x ∈ Finset.range n, (f (x + 1) + -f x) • ∑ i ∈ Finset.range (x + 1), z i

Proof state:
case refine_2
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
n : ℕ
⊢ ‖(f (n + 1) + -f n) • (Finset.range (n + 1)).sum z‖ ≤ b * |f (n + 1) - f n|

Tactic:
rw [norm_smul, mul_comm]

Result:
case refine_2
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
n : ℕ
⊢ ‖(Finset.range (n + 1)).sum z‖ * ‖f (n + 1) + -f n‖ ≤ b * |f (n + 1) - f n|

Proof state:
case refine_1
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => f n + -f 0

Tactic:
exact (Tendsto.cauchySeq hf0).add_const

Result:
no goals

Proof state:
case refine_1
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ i ∈ Finset.range n, b * |f (i + 1) - f i|

Tactic:
simp_rw [abs_of_nonneg (sub_nonneg_of_le (hfa (Nat.le_succ _))), ← mul_sum]

Result:
case refine_1
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => b * ∑ i ∈ Finset.range n, (f i.succ - f i)

Proof state:
case refine_1
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => ∑ i ∈ Finset.range n, (f i.succ - f i)

Tactic:
simp_rw [sum_range_sub, sub_eq_add_neg]

Result:
case refine_1
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Monotone f
hf0 : Tendsto f atTop (𝓝 0)
hgb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
⊢ CauchySeq fun n => f n + -f 0