Let $x,y$ be sets. We use the following notation:
- $x\simeq y$ means that there is a bijection $\varphi:x\to y$, and
- $x\leq y$ means that there is an injection $\iota:x\to y$.
The Weak Power Hypothesis says
(WPH) Whenever ${\cal P}(x)\simeq {\cal P}(y)$ then $x\simeq y$.
Consider the statement
(WPH$_\leq$) Whenever ${\cal P}(x) \leq {\cal P}(y)$ then $x\leq y$.
Using the theorem of Cantor-Bernstein-Schroeder we can show in ${\sf (ZF)}$ that (WPH$_\leq$) implies (WPH).
Question. In ${\sf (ZF)}$, does (WPH) imply (WPH$_\leq$)?
Notes.
As Joel David Hawkins notes in the comment section, (WPH) and (WPH$_\leq$) are equivalent in ${\sf (ZFC)}$.
There are models of ${\sf (ZFC)}$ in which (WPH) does not hold - but interestingly, it appears to be open whether (WPH) implies (AC).