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How much of Zorn's lemma can be saved if we assume only ZF+DC without full choice? More precisely: assume we have a partially ordered (inductive) set which is of size continuum. Then can we apply Zorn's lemma in this case assuming only dependent choice?

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It is a theorem of Wolk [1] that for every infinite $\kappa$ the axiom $\sf DC_\kappa$ is equivalent to the following statement:

If $(P,\leq)$ is a partially ordered set in which every well-ordered chain has order type $<\kappa$ and has an upper-bound, then $P$ contains a maximal element.

In the case of $\kappa=\omega$ this means that every partial order in which every well-ordered chain is finite and bounded has a maximal element.

Note that there is no limitation on the cardinality of $P$. Only on its well-ordered chains.


Bibliography:

  1. Elliot S. Wolk, On the principle of dependent choices and some forms of Zorn’s lemma, Canad. Math. Bull. 26 (1983), no. 3, 365–367.
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  • $\begingroup$ What can we save ($DC_{\omega}$ or $DC_{\aleph_1}$) in Solovay's model? $\endgroup$ – user48900 Apr 26 '13 at 21:00
  • $\begingroup$ Solovay's model satisfies $\sf DC_\omega$. When we say "save" we mean that we can construct a model in such way that $\sf DC_\omega$ (or otherwise) is retained. It cannot satisfy $\sf DC_{\aleph_1}$ as that implies there is a non-measurable set. $\endgroup$ – Asaf Karagila Apr 26 '13 at 21:02

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