Partition Principle and (DC) – Set Theory Implications

axiom-of-choicelo.logicset-theory

For sets $x, y$ we write $x\leq y$, if there is an injection $\iota: x \to y$, and we write $x \leq^* y$ if either $x = \emptyset$ or there is a surjection $s: y \to x$. In ${\sf (ZF)}$ we have that $x \leq y$ implies $y \leq^* y$.

Consider the following statements:

Partition principle (PP): For all sets $x, y$ we have that $x \leq^* y$ implies $x\leq y$.

Dual Cantor-Bernstein (CB)*: For all sets $x,y$, if $x\leq^* y$ and $y \leq^* x$, then there is a bijection $\varphi: x\to y$.

Via the "normal" Cantor-Bernstein theorem, which a theorem of ${\sf (ZF)}$, we can show that (PP) imples (CB)* in ${\sf (ZF)}$.

It seems to be open whether (CB)* implies the Axiom of Dependent Choice (DC). Since (PP) is stronger than (CB)*, this begs the question:

Does (PP) imply (DC) in ${\sf (ZF)}$?

Best Answer

Yes. This is a combination of facts.

  1. $\sf PP$ implies that if a set $X$ can be mapped onto an ordinal $\alpha$, then $\alpha$ injects into $X$. In other words, it implies that $\aleph^*(X)=\aleph(X)$ for any set $X$.

  2. $\sf AC_{WO}$, that is the axiom of choice from families of sets indexed by an ordinal, is equivalent to the statement "For every $X$, $\aleph^*(X)=\aleph(X)$".

  3. $\sf AC_{WO}$ implies $\sf DC$. This is due to the fact that if $T$ is a tree of height $\omega$ without cofinal branches, we can consider the various well-orderable subtrees of $T$, define a rank function on those and use that to define a rank function on $T$. Then, using $\sf AC_{WO}$ we can show that it is impossible that $T$ itself is not truly well-founded, which would mean that it has a maximal element.