Every proper ideal is contained in a maximal ideal
The corresponding statement is not true in a number of other algebraic structure, primarily because the condition of being proper cannot be tested through the presence or absence of a single element. For instance, it is not in general true for modules over a fixed commutative ring, and it is not true in general for groups. The key difference in rings is that the presence or absence of a single element can be used to test for properness.
The proof uses the axiom of choice. We present here three slightly different formulations: transfinite induction, Zorn's lemma and the Hausdorff maximality principle.
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