theorem activation_two_level {k10 k21 : ℝ} (hk10 : 0 < k10) (hk21 : 0 < k21) :
1 < Real.sqrt (k10 * k21) ↔ 1 < k10 * k21 := by
constructor
· intro h
have hsq := sq_lt_sq' (by linarith : -Real.sqrt (k10 * k21) < 1) h
rw [one_pow, sq_sqrt (mul_nonneg (le_of_lt hk10) (le_of_lt hk21))] at hsq
exact hsq
· intro h
rw [show (1 : ℝ) = Real.sqrt 1 from (Real.sqrt_one).symm]
exact Real.sqrt_lt_sqrt (by linarith) hTheorems 5-6, Proposition 3: Next-Generation Matrix and Activation Threshold