Proposition 3.6. Let X be a self-distributive BE-algebra and I an ideal of
X. Then Iw is an ideal of an CI-algebra X.
Proof. Let x ∈ X and a ∈ Iw. Then we have w ∗ a ∈ I, and so w ∗ (x ∗ a)= (w ∗ x) ∗ (w ∗ a) ∈ X ∗ I ⊆ I from (I1). This implies x ∗ a ∈ Iw. Now let a, b ∈ Iw and x ∈ X. Then we obtain w ∗ a ∈ I and w ∗ b ∈ I. Thus we get w∗((a∗(b∗x))∗x)=(w∗((a∗(b∗x))))∗(w∗x)=((w∗a)∗(w∗(b∗x)))∗(w∗x)= ((w ∗ a) ∗((w ∗ b) ∗(w ∗x)))∗ (w ∗x)=((w ∗a)∗((w ∗ b) ∗(w ∗x)))∗ (w ∗x) ∈ I by (I2). This implies (a ∗ (b ∗ x)) ∗ x ∈ Iw. This completes the proof.