The four principles are:
- Axiom of choice: for every family of nonempty sets, there exists a function on with for all .
- Cardinal comparability principle: for any sets , there is an injection or an injection .
- Well-ordering theorem: every set admits a well-order.
- Zorn's lemma: every nonempty partially ordered set in which every chain has an upper bound has a maximal element.
In the last assertion the upper bound belongs to the partially ordered set; it need not belong to the chain.
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