theorem isoquant_umbilic (J : ℕ) (ρ c : ℝ) (v : Fin J → ℝ) : secondFundamentalFormQF J ρ c v = cesPrincipalCurvature J ρ c * vecNormSq J v := rfl
thesis/CESProofs/Foundations/IsoquantGeometry.lean:204
Differential Geometry of CES Isoquants (Gap #6)