theorem valueFunctionDeriv_pos {J ρ c : ℝ}
(hJ : 1 < J) (hρ0 : 0 < ρ) (hρ1 : ρ < 1) (hc : 0 < c) :
0 < valueFunctionDeriv J ρ c := by
simp only [valueFunctionDeriv]
have hJpos : 0 < J := by linarith
apply add_pos
· exact mul_pos (pigouvian_subsidy_pos hJpos hρ1) (perCapitaBenefit_pos hJpos hc)
· apply mul_pos (curvatureKReal_pos hJ hρ1)
apply mul_pos (mul_pos _ (rpow_pos_of_pos hJpos _)) hc
rw [sub_pos, lt_div_iff₀ hρ0]; linarithPaper 1c: Formal Calculus on K(J) and V(J)