Solution
= Solution
Hessenberg's theorem in ZF states that every infinite well-orderable cardinal satisfies $\kappa\times\kappa=\kappa$; equivalently $|\alpha\times\alpha|=|\alpha|$ for every infinite ordinal $\alpha$.
= Solution
Hessenberg's theorem in ZF states that every infinite well-orderable cardinal satisfies $\kappa\times\kappa=\kappa$; equivalently $|\alpha\times\alpha|=|\alpha|$ for every infinite ordinal $\alpha$.