phi_irrational

formal source

  58
  59    Proof: Our φ equals Mathlib's golden ratio, which is proven irrational
  60    via the irrationality of √5 (5 is prime, hence not a perfect square). -/
  61theorem phi_irrational : Irrational phi := by
  62  -- Our phi equals Mathlib's goldenRatio
  63  have h_eq : phi = Real.goldenRatio := rfl
  64  rw [h_eq]
  65  exact Real.goldenRatio_irrational
  66
  67/-! ### φ power bounds -/
  68
  69/-- Key identity: φ² = φ + 1 (from the defining equation x² - x - 1 = 0). -/
  70lemma phi_sq_eq : phi^2 = phi + 1 := by
  71  simp only [phi]
  72  have h5_pos : (0 : ℝ) ≤ 5 := by norm_num
  73  have hsq : (Real.sqrt 5)^2 = 5 := Real.sq_sqrt h5_pos
  74  ring_nf
  75  linear_combination (1/4) * hsq
  76
  77/-- Tighter lower bound: φ > 1.5 (since √5 > 2, so (1 + √5)/2 > 1.5). -/
  78lemma phi_gt_onePointFive : (1.5 : ℝ) < phi := by
  79  simp only [phi]
  80  have h5 : (2 : ℝ) < Real.sqrt 5 := by
  81    have h : (2 : ℝ)^2 < 5 := by norm_num
  82    rw [← Real.sqrt_sq (by norm_num : (0 : ℝ) ≤ 2)]
  83    exact Real.sqrt_lt_sqrt (by norm_num) h
  84  linarith
  85
  86/-- Tighter upper bound: φ < 1.62 (since √5 < 2.24). -/
  87lemma phi_lt_onePointSixTwo : phi < (1.62 : ℝ) := by
  88  simp only [phi]
  89  have h5 : Real.sqrt 5 < (2.24 : ℝ) := by
  90    have h : (5 : ℝ) < (2.24 : ℝ)^2 := by norm_num
  91    have h24_pos : (0 : ℝ) ≤ 2.24 := by norm_num