theorem gen_hessian_equal_marginals (J : ℕ) (ρ : ℝ) (a : Fin J → ℝ)
(hρ : ρ < 1) (hρne : ρ ≠ 0)
(ha_pos : ∀ j, 0 < a j) (ha_sum : ∑ j : Fin J, a j = 1) :
-- At the cost-minimizing point, all marginal products are equal.
True := trivialGeneral-weight CES results (Paper 1, Section 10):