not an NC-Ideal of X, since η(0 ∗ 1) > η(1).
Proposition 3.7. If (X, η) is N-Ideal of X with x ≤ y for any x, y ∈ X
then η(x) ≤ η(y). (i.e) η is order-preserving.
Proof. Let x, y ∈ X such that x ≤ y. Then x ∗ y = 0. Thus
η(x) ≤ η(x ∗ y) ∨ η(y) = η(0) ∨ η(y) = η(y).