have h_nonneg : 0 < (2 * n) ^ (2 / 3) := by apply Real.rpow_pos_of_pos; linarith
n : ℝ
hn : 0 < n
⊢ 2 * n < 20 * (2 * n) ^ (2 / 3) ↔ n < 4000
n : ℝ
hn : 0 < n
h_nonneg : 0 < (2 * n) ^ (2 / 3)
⊢ 2 * n < 20 * (2 * n) ^ (2 / 3) ↔ n < 4000