Proposition 2.11. Let x,y,z be any element in a KK-algebra X. Then (1) ((x∗y)∗y)∗y = x∗y. (2) (x∗y)∗0=( x∗0)∗(y∗0). Proof. (1) From theorem 2.3(2) and theorem 2.7(1) , (((x∗y)∗y)∗y)∗ (x ∗ y) ≤ x ∗((x ∗ y)∗ y)=0 . Thus (((x ∗y)∗ y)∗y)∗ (x ∗ y)=0 . Since (x ∗ y)∗ (((x ∗ y)∗y)∗ y) = (( x ∗ y)∗y)∗ ((x ∗ y)∗y)=0 . So, by KK-3, (x∗y)∗y = x∗y. (2) Since (x∗0)∗(y∗0) = (x∗0)∗(y∗((x∗y)∗(x∗y))) = (x∗0)∗((x∗y)∗(y∗ (x∗y))) = (x∗0)∗((x∗y)∗(x∗(y∗y))) = (x∗y)∗((x∗0)∗(x∗0)) = (x∗y)∗0. The proof is complete.
In this paper we will denote N for the set of all nonnegative integers, i.e.,