Proof. Assume that (X;∗,0) is a KK-algebra. From definition of KKalgebra, (1) and (4) holds. Then we see that x∗((x∗y)∗y) = (0∗x)∗((x∗ y)∗(0∗y)) = 0, and x∗x =0∗(x∗x) = (0∗0)∗((0∗x)∗(0∗x)) = 0, so (2) and (3) holds. Conversely, we need to show KK-2. By (1), (2) and (3), we see that ((0∗x )∗x)∗0 = ((0∗x)∗x)∗(0∗((0∗x)∗x)) = ((0∗x)∗x)∗((x∗x)∗((0∗x)∗x)) = 0. And since 0∗((0∗x)∗x)=0 . From (4), it follows that (0∗x)∗x = 0 and x∗(0∗x)=x∗((x∗x)∗x)=0 . Therefore 0∗x = x, proving our theorem.