theorem static_vri_cross {Jn Jm : ℕ}
(P_n : Fin Jn → ℝ) (P_m : Fin Jm → ℝ)
(x_n : Fin Jn → ℝ) (x_m : Fin Jm → ℝ) :
(∑ j, ∑ k, P_n j * P_m k * x_n j * x_m k) =
(∑ j, P_n j * x_n j) * (∑ k, P_m k * x_m k) := by
have h : ∀ j k, P_n j * P_m k * x_n j * x_m k =
(P_n j * x_n j) * (P_m k * x_m k) := by intros; ring
simp_rw [h, ← Finset.mul_sum, ← Finset.sum_mul]Results 8-16: Variance-Response Identity and Early Warning