theorem herfindahl_equal_weights (J : ℕ) (hJ : 0 < J) :
herfindahlIndex (equalWeights J) = 1 / (J : ℝ) := by
simp only [herfindahlIndex, equalWeights]
simp only [Finset.sum_const, Finset.card_fin, nsmul_eq_mul]
field_simpHerfindahl Dynamics: Entry, Exit, and Merger Effects on Concentration