English

Cantor's theorem may fail for finitary partitions

Logic 2023-09-04 v1

Abstract

A partition is finitary if all its members are finite. For a set AA, B(A)\mathscr{B}(A) denotes the set of all finitary partitions of AA. It is shown consistent with ZF\mathsf{ZF} (without the axiom of choice) that there exist an infinite set AA and a surjection from AA onto B(A)\mathscr{B}(A). On the other hand, we prove in ZF\mathsf{ZF} some theorems concerning B(A)\mathscr{B}(A) for infinite sets AA, among which are the following: (1) If there is a finitary partition of AA without singleton blocks, then there are no surjections from AA onto B(A)\mathscr{B}(A) and no finite-to-one functions from B(A)\mathscr{B}(A) to AA. (2) For all nωn\in\omega, An<B(A)|A^n|<|\mathscr{B}(A)|. (3) B(A)seq(A)|\mathscr{B}(A)|\neq|\mathrm{seq}(A)|, where seq(A)\mathrm{seq}(A) is the set of all finite sequences of elements of AA.

Keywords

Cite

@article{arxiv.2309.00235,
  title  = {Cantor's theorem may fail for finitary partitions},
  author = {Guozhen Shen},
  journal= {arXiv preprint arXiv:2309.00235},
  year   = {2023}
}

Comments

19 pages

R2 v1 2026-06-28T12:09:57.907Z