A subset A of a KK-algebra X is called closed of X if x ∗ y ∈ A whenever x,y ∈ A. A non-empty subset A of a KK-algebra X is called an ideal of X if it satisfies the following conditions:
(I-1) 0 ∈ A
(I-2) for any x,y ∈ X, x ∗ y ∈ A and x ∈ A imply y ∈ A.