Theorem 3.9. If (X, η) is N-Ideal of X then for any
x, a1, a2, · · · , an ∈ X and (· · ·((x ∗ a1) ∗ a2) ∗ · · ·)) ∗ an = 0 implies η(x) ≤
η(a1) ∨ η(a2) ∨ · · · ∨ η(an).
Theorem 3.10. Any N-Ideal of X is an N-subalgebra ofX.
The converse of the theorem 3.10 is not be true in general, as seen from the
following
Example 3.11. For the N-subalgebra in example 2.9. Take x = 2 and y =
4. We have η(2) = −0.2 and η(2 ∗ 4)∨ η(4) = η(3)∨ η(4) = −0.8∨ −0.8 = −0.8
⇒ η(x) > η(x ∗ y) ∨ η(y) and so it is not N-Ideal.
The following gives a sufficient condition for an N-subalgebra to be an Nideal