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Consider the following statements about a lattice L :

(i) Every ideal of L is a prime ideal.
(ii) L is a chain.

Then which of the following is correct ?

  • A.(I) implies (II) but not conversely
  • B.(II) implies (I) but not conversely
  • C.(I) if and only if (II)
  • D.Neither (I) implies (II) nor (II) implies (I)

Answer: C

The answer is (I) if and only if (II).

If L is a chain and \(a \wedge b \in I\) for an ideal I, then \(a \wedge b\) is the smaller of \(a\) and \(b\), so one of them lies in I: every ideal is prime.

Conversely, suppose every ideal is prime but \(a\) and \(b\) are incomparable. The principal ideal \((a \wedge b]\) contains \(a \wedge b\), so being prime it contains \(a\) or \(b\). If it contains \(a\), then \(a \le a \wedge b \le b\), contradicting incomparability; similarly for \(b\). So L is a chain. Each condition implies the other.

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