theorem extracted_formal_statement_3.{u_1} {E : Type u_1} [inst : NormedAddCommGroup E] [inst_1 : InnerProductSpace ℝ E]
  [inst_2 : CompleteSpace E] [inst_3 : ProperSpace E] {x₀ : E} {f : E → ℝ} {f' : E → E} {h : E → ℝ}
  {alg : proximal_gradient_method f h f' x₀} (th : ContinuousOn (proximal_gradient_method.t f h f' x₀ • h) univ)
  (th' : ConvexOn ℝ univ (proximal_gradient_method.t f h f' x₀ • h)) :
  let Gt := fun x =>
    (1 / proximal_gradient_method.t f h f' x₀) •
      (x -
        prox_point_c' (proximal_gradient_method.t f h f' x₀ • h) (x - proximal_gradient_method.t f h f' x₀ • f' x) th
          th');
  ∀ (z : E),
    z - proximal_gradient_method.t f h f' x₀ • Gt z =
        prox_point_c' (proximal_gradient_method.t f h f' x₀ • h) (z - proximal_gradient_method.t f h f' x₀ • f' z) th
          th' →
      proximal_gradient_method.t f h f' x₀ ≠ 0 := sorry

State before:
E : Type u_1
inst✝³ : NormedAddCommGroup E
inst✝² : InnerProductSpace ℝ E
inst✝¹ : CompleteSpace E
inst✝ : ProperSpace E
x₀ : E
f : E → ℝ
f' : E → E
h : E → ℝ
alg : proximal_gradient_method f h f' x₀
k : ℕ+
th : ContinuousOn (proximal_gradient_method.t f h f' x₀ • h) univ
th' : ConvexOn ℝ univ (proximal_gradient_method.t f h f' x₀ • h)
Gt : E → E :=
  fun x =>
    (1 / proximal_gradient_method.t f h f' x₀) •
      (x -
        prox_point_c' (proximal_gradient_method.t f h f' x₀ • h) (x - proximal_gradient_method.t f h f' x₀ • f' x) th
          th')
φ : E → ℝ := fun x => f x + h x
z : E
⊢ z - proximal_gradient_method.t f h f' x₀ • Gt z =
    prox_point_c' (proximal_gradient_method.t f h f' x₀ • h) (z - proximal_gradient_method.t f h f' x₀ • f' z) th th'

Tactic: simp [Gt]
    rw [smul_inv_smul₀, ← sub_add]
    simp

State after:
[case ha
 E : Type u_1
 inst✝³ : NormedAddCommGroup E
 inst✝² : InnerProductSpace ℝ E
 inst✝¹ : CompleteSpace E
 inst✝ : ProperSpace E
 x₀ : E
 f : E → ℝ
 f' : E → E
 h : E → ℝ
 alg : proximal_gradient_method f h f' x₀
 k : ℕ+
 th : ContinuousOn (proximal_gradient_method.t f h f' x₀ • h) univ
 th' : ConvexOn ℝ univ (proximal_gradient_method.t f h f' x₀ • h)
 Gt : E → E :=
   fun x =>
     (1 / proximal_gradient_method.t f h f' x₀) •
       (x -
         prox_point_c' (proximal_gradient_method.t f h f' x₀ • h) (x - proximal_gradient_method.t f h f' x₀ • f' x) th
           th')
 φ : E → ℝ := fun x => f x + h x
 z : E
 ⊢ proximal_gradient_method.t f h f' x₀ ≠ 0]

Relevant lemmas:
sub_add : ∀ {α : Type u_1} [inst : SubtractionCommMonoid α] (a b c : α), a - b + c = a - (b - c)
smul_inv_smul₀ : ∀ {α : Type u_4} {β : Type u_5} [inst : GroupWithZero α] [inst_4 : MulAction α β] {a : α},
  a ≠ 0 → ∀ (x : β), a • a⁻¹ • x = x