theorem principalCurvature_trichotomy (hJ : 2 ≤ J) {c : ℝ} (hc : 0 < c) (ρ : ℝ) :
(ρ < 1 → 0 < cesPrincipalCurvature J ρ c) ∧
(ρ = 1 → cesPrincipalCurvature J ρ c = 0) ∧
(1 < ρ → cesPrincipalCurvature J ρ c < 0) := by
refine ⟨fun hρ => curvature_pos hJ hρ hc, fun hρ => ?_, fun hρ => ?_⟩
· simp [cesPrincipalCurvature, hρ]
· simp only [cesPrincipalCurvature]
apply div_neg_of_neg_of_pos
· linarith
· apply mul_pos hc
exact Real.sqrt_pos_of_pos (Nat.cast_pos.mpr (by omega))Differential Geometry of CES Isoquants (Gap #6)