private lemma harmonic_mean_ge_half_min {a b : ℝ} (ha : 0 < a) (hb : 0 < b) :
min a b / 2 ≤ a * b / (a + b) := by
rw [div_le_div_iff₀ two_pos (by linarith : 0 < a + b)]
by_cases h : a ≤ b
· rw [min_eq_left h]
nlinarith [mul_le_mul_of_nonneg_left h ha.le]
· push_neg at h
rw [min_eq_right h.le]
nlinarith [mul_le_mul_of_nonneg_left h.le hb.le]Superadditivity of CES (Paper 1, Section 6):