phi_eq_one_add_inv_phi

formal source

  44  gapAffineLogR a b c (Z : ℝ)
  45
  46/-- `φ = 1 + 1/φ` (golden ratio identity). -/
  47lemma phi_eq_one_add_inv_phi : phi = 1 + (1 : ℝ) / phi := by
  48  have hne : phi ≠ 0 := phi_ne_zero
  49  calc
  50    phi = phi ^ 2 / phi := by field_simp [hne]
  51    _ = (phi + 1) / phi := by simp [phi_sq_eq]
  52    _ = 1 + (1 : ℝ) / phi := by field_simp [hne]
  53
  54lemma one_add_inv_phi_eq_phi : 1 + (1 : ℝ) / phi = phi :=
  55  phi_eq_one_add_inv_phi.symm
  56
  57lemma log_one_add_inv_phi_eq_log_phi : Real.log (1 + phi⁻¹) = Real.log phi := by
  58  have hshift : (1 + phi⁻¹ : ℝ) = phi := by
  59    simpa [one_div] using one_add_inv_phi_eq_phi
  60  simp [hshift]
  61
  62/-! ## Step 1: g(0) = 0 forces c = 0 -/
  63
  64lemma zero_normalization_forces_offset
  65    {a c : ℝ}
  66    (h0 : gapAffineLogR a phi c 0 = 0) :
  67    c = 0 := by
  68  simpa [gapAffineLogR] using h0
  69
  70/-! ## Step 2: g(1) = 1 forces a = 1/log(φ) (given c = 0 and b = φ) -/
  71
  72lemma unit_step_forces_log_scale
  73    {a c : ℝ}
  74    (h0 : gapAffineLogR a phi c 0 = 0)
  75    (h1 : gapAffineLogR a phi c 1 = 1) :
  76    a = 1 / Real.log phi := by
  77  have hc : c = 0 := zero_normalization_forces_offset h0