Set Theory – Proving Well-Orderability of 2^X Without Axiom of Choice

axiom-of-choiceorder-theoryset-theorywell-orders

Without appealing to the axiom of choice, it can be shown that (Proposition:) if $X$ is well-orderable, then $2^X$ is totally-orderable.

Question: can we show the stronger result that if $X$ is well-orderable, then so too is $2^X$?

Proof of Proposition. Pick any well-ordering of $X$. Then the lexicographic order totally orders $2^X$.

More explicitly: for any two $f,g \in 2^X$, define $f < g$ iff

  1. there exists $x \in X$ such that $f(x) \neq g(x)$, and
  2. if $x \in X$ is minimal such that $f(x) \neq g(x)$, then $f(x)=0$ and $g(x)=1$.

It can be shown that $<$ totally-orders $2^X$.

Best Answer

No.

The axiom "If $X$ can be well-ordered then $2^X$ can be well-ordered" implies the axiom of choice in $\sf ZF$. In $\sf ZFA$ or $\sf ZF-Reg$ this is no longer true, though.

To see more details see the first part of Jech, The Axiom of Choice Chapter 9.

One very interesting observation about the fact that in $\sf ZFA$ this statement does not prove the axiom of choice, is that if $\psi(X)$ is a statement in which all the quantifiers are either bounded in $y$ or bounded in $\mathcal P(y)$ (where $y$ is a variable, of course), and in $\sf ZF$ we have that $\forall X\psi(X)$ implies $\sf AC$, then in $\sf ZFA$ we have that $\forall X\psi(X)$ implies that the power set of a well-ordered set is well-orderable. (This is the first problem in the aforementioned chapter 9.)

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