0 < i ∧ j < 0 ∨ i < 0 ∧ 0 < j ⊦ i * j < 0
⊦ 0 < i ∧ j < 0 ∨ i < 0 ∧ 0 < j, i * j = 1 ↔ abs(j) = 1 ∧ i = j, 0 < i ∧ 1 < j
i * j < 0 ⊦ i * j = 1 ↔ abs(j) = 1 ∧ i = j, 0 < i ∧ 1 < j
Used terms: i, j