theorem extracted_formal_statement_75.{u_1} {E : Type u_1} [inst : NormedAddCommGroup E] [inst_1 : InnerProductSpace ℝ E]
  {f : E → ℝ} (xm x₀ : E) {alg : subgradient_method f x₀} (k : ℕ)
  (h' : ∀ ⦃x g : E⦄, g ∈ SubderivAt f x → ‖g‖ ≤ ↑(subgradient_method.G f x₀)) (k₀ : ¬k = 0)
  (heq :
    (Set.range fun x => f (subgradient_method.x f x₀ ↑x)) =
      {x | ∃ i ∈ Finset.range (k + 1), f (subgradient_method.x f x₀ i) = x})
  (h₁ :
    ∀ ⦃i : ℕ⦄,
      i ≥ 0 ∧ i ≤ k →
        ‖subgradient_method.x f x₀ (i + 1) - xm‖ ^ 2 ≤
          ‖subgradient_method.x f x₀ i - xm‖ ^ 2 -
              2 * subgradient_method.a f x₀ i *
                (sInf {x | ∃ i ∈ Finset.range (k + 1), f (subgradient_method.x f x₀ i) = x} - f xm) +
            ↑(subgradient_method.G f x₀) ^ 2 * subgradient_method.a f x₀ i ^ 2)
  (h₁' :
    ∀ ⦃i : ℕ⦄,
      i ≥ 0 ∧ i ≤ k →
        subgradient_method.a f x₀ i *
            (2 * (sInf {x | ∃ i ∈ Finset.range (k + 1), f (subgradient_method.x f x₀ i) = x} - f xm)) ≤
          ‖subgradient_method.x f x₀ i - xm‖ ^ 2 - ‖subgradient_method.x f x₀ (i + 1) - xm‖ ^ 2 +
            ↑(subgradient_method.G f x₀) ^ 2 * subgradient_method.a f x₀ i ^ 2)
  (i : ℕ) (hi : i ∈ Finset.range (k + 1)) (this : i < k + 1) : i ≤ k := sorry

State before:
case h.a.right
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : InnerProductSpace ℝ E
f : E → ℝ
xm x₀ : E
alg : subgradient_method f x₀
k : ℕ
h' : ∀ ⦃x g : E⦄, g ∈ SubderivAt f x → ‖g‖ ≤ ↑(subgradient_method.G f x₀)
k₀ : ¬k = 0
heq :
  (Set.range fun x => f (subgradient_method.x f x₀ ↑x)) =
    {x | ∃ i ∈ Finset.range (k + 1), f (subgradient_method.x f x₀ i) = x}
h₁ :
  ∀ ⦃i : ℕ⦄,
    i ≥ 0 ∧ i ≤ k →
      ‖subgradient_method.x f x₀ (i + 1) - xm‖ ^ 2 ≤
        ‖subgradient_method.x f x₀ i - xm‖ ^ 2 -
            2 * subgradient_method.a f x₀ i *
              (sInf {x | ∃ i ∈ Finset.range (k + 1), f (subgradient_method.x f x₀ i) = x} - f xm) +
          ↑(subgradient_method.G f x₀) ^ 2 * subgradient_method.a f x₀ i ^ 2
h₁' :
  ∀ ⦃i : ℕ⦄,
    i ≥ 0 ∧ i ≤ k →
      subgradient_method.a f x₀ i *
          (2 * (sInf {x | ∃ i ∈ Finset.range (k + 1), f (subgradient_method.x f x₀ i) = x} - f xm)) ≤
        ‖subgradient_method.x f x₀ i - xm‖ ^ 2 - ‖subgradient_method.x f x₀ (i + 1) - xm‖ ^ 2 +
          ↑(subgradient_method.G f x₀) ^ 2 * subgradient_method.a f x₀ i ^ 2
i : ℕ
hi : i ∈ Finset.range (k + 1)
this : i < k + 1
⊢ i ≤ k

Tactic: apply (Nat.lt_add_one_iff).mp this

State after:

Relevant lemmas:
Nat.lt_add_one_iff : ∀ {m n : ℕ}, m < n + 1 ↔ m ≤ n