Proof. Assume that I is an ideal of KK-algebra X and x,y,z ∈ X. (1) ⇒ (2) Let I be an q-ideal of X and (x∗0)∗y) ∈ I. Since 0 ∈ I, by (I-3), x ∗ y ∈ I. (2) ⇒ (3) Suppose that (2) holds and (x∗y)∗z ∈ I. We see that (x∗y)∗z)∗ ((x∗0)∗(y∗z)) ≤ (x∗0)∗(y∗(x∗y)) = (x∗0)∗(x∗(y∗y)) = (x∗0)∗(x∗0) = 0 ∈ I. Since I is an ideal of X, so ((x ∗ 0) ∗ (y ∗ z)) ∈ I. By (2), so x ∗ (y ∗ z) ∈ I. (3) ⇒ (1) Let (x∗y)∗z ∈ I and y ∈ I. From (3), we obtain that x∗(y∗z) ∈ I. Thus y ∗ (x ∗ z) ∈ I by (P-3). Since y ∈ I and I is an ideal, hence x ∗ z ∈ I, proving that I is a q-ideal of X