theorem cesPriceIndex_eq_sigma {ρ : ℝ} (hρ1 : ρ ≠ 1) {p : Fin J → ℝ} :
cesPriceIndex J ρ p =
(∑ j : Fin J, (p j) ^ (1 - elasticityOfSubstitution ρ)) ^
(1 / (1 - elasticityOfSubstitution ρ)) := by
simp only [cesPriceIndex, unnormCES]
congr 1
· apply Finset.sum_congr rfl; intro j _
congr 1; exact dualExponent_eq_sigma hρ1
· congr 1; exact dualExponent_eq_sigma hρ1Economics extensions for CES formalization: