theorem cesHessianQF_on_perp (hJ : 0 < J) (ρ c : ℝ) (hc : 0 < c)
(v : Fin J → ℝ) (hv : orthToOne J v) :
cesHessianQF J ρ c v = cesEigenvaluePerp J ρ c * vecNormSq J v := by
simp only [cesHessianQF, cesEigenvaluePerp, orthToOne, vecSum, vecNormSq] at *
rw [hv]
have hJne : (↑J : ℝ) ≠ 0 := Nat.cast_ne_zero.mpr (by omega)
have hcne : c ≠ 0 := ne_of_gt hc
field_simp
ringGradient and Hessian of CES at the symmetric point.