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    Maximal ideals are prime; the converse fails

    P-AMD-2NXEX5LK

    Area
    Algebra
    Topics
    Ideals, Maximal Ideals, Prime Ideals
    Status
    draft

    Maximal \(\implies\) prime, but generally not the converse.

    Solution

    Assume rings are commutative with identity.

    1. Every maximal ideal is prime.

    Let \(\mathfrak m\) be maximal. Then \(R/\mathfrak m\) is a field, hence an integral domain. An ideal is prime exactly when its quotient ring is an integral domain. Therefore \(\mathfrak m\) is prime.

    2. The converse fails in general.

    In \(\ZZ\), the zero ideal \((0)\) is prime because \(\ZZ\) is an integral domain. But \((0)\) is not maximal, since \[ (0)\subsetneq (2)\subsetneq \ZZ. \] Thus a prime ideal need not be maximal.