prime(i), 0 < i, 1 < i ⊦ prime(i) ↔ (∀ j. prime(j) ∧ j * j ≤ i → ¬ j | i)
0 < i, 1 < i ⊦ prime(i), prime(i) ↔ (∀ j. prime(j) ∧ j * j ≤ i → ¬ j | i)
Used formulas: prime(i)