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

Tactic:
have hfa' : Monotone fun n ↦ -f n := fun _ _ hab ↦ neg_le_neg <| hfa hab

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

Proof state:
case h.e'_5.h
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Antitone f
hf0 : Tendsto f atTop (𝓝 0)
hzb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
hfa' : Monotone fun n => -f n
hf0' : Tendsto (fun n => -f n) atTop (𝓝 0)
x✝ : ℕ
⊢ ∑ i ∈ Finset.range x✝, f i • z i = (-fun n => ∑ i ∈ Finset.range n, -f i • z i) x✝

Tactic:
simp

Result:
no goals

Proof state:
case h.e'_5.h.e'_3
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Antitone f
hf0 : Tendsto f atTop (𝓝 0)
hzb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
hfa' : Monotone fun n => -f n
⊢ 0 = -0

Tactic:
norm_num

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 : Antitone f
hf0 : Tendsto f atTop (𝓝 0)
hzb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
hfa' : Monotone fun n => -f n
hf0' : Tendsto (fun n => -f n) atTop (𝓝 0)
⊢ CauchySeq fun n => ∑ i ∈ Finset.range n, f i • z i

Tactic:
convert (hfa'.cauchySeq_series_mul_of_tendsto_zero_of_bounded hf0' hzb).neg

Result:
case h.e'_5.h
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Antitone f
hf0 : Tendsto f atTop (𝓝 0)
hzb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
hfa' : Monotone fun n => -f n
hf0' : Tendsto (fun n => -f n) atTop (𝓝 0)
x✝ : ℕ
⊢ ∑ i ∈ Finset.range x✝, f i • z i = (-fun n => ∑ i ∈ Finset.range n, -f i • z i) x✝

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

Tactic:
convert hf0.neg

Result:
case h.e'_5.h.e'_3
E : Type u_2
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
b : ℝ
f : ℕ → ℝ
z : ℕ → E
hfa : Antitone f
hf0 : Tendsto f atTop (𝓝 0)
hzb : ∀ (n : ℕ), ‖∑ i ∈ Finset.range n, z i‖ ≤ b
hfa' : Monotone fun n => -f n
⊢ 0 = -0

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

Tactic:
have hf0' : Tendsto (fun n ↦ -f n) atTop (𝓝 0) := by
  convert hf0.neg
  norm_num

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