Proposition 3.5. Let X be a self-distributive CI-algebra and I an ideal of X. Then Iw is a subalgebra of an CI-algebra X.
Proof. Let a, b ∈ Iw. Then w ∗ a ∈ I and w ∗ b ∈ I, and so w ∗ (a ∗ b)= (w ∗ a) ∗ (w ∗ b) ⊆ I ∗ I ⊆ X ∗ I ⊆ I. This implies a ∗ b ∈ Iw.