Problem #5

Does the Partition Principle imply the Axiom of Choice?

Open!!!Very high impact — resolution would be of award-level significance

The Partition Principle (PP\mathsf{PP}) states: if there is a surjection from a set AA onto a set BB (equivalently, BB admits a partition into at most A|A| nonempty blocks), then there is an injection from BB into AA. In ZF\mathsf{ZF} (without choice), PP\mathsf{PP} follows from the Axiom of Choice (AC\mathsf{AC}), but whether

ZF+PP    AC\mathsf{ZF} + \mathsf{PP} \implies \mathsf{AC}

is one of the oldest open problems concerning choice principles, going back to the early 1900s.

Reference for the problem statement

Herman Rubin and Jean E. Rubin, Equivalents of the Axiom of Choice II, North-Holland, 1985

Definitions

  • ZF\mathsf{ZF}: Zermelo–Fraenkel set theory without the Axiom of Choice.
  • Partition Principle (PP\mathsf{PP}): for sets A,BA, B, if there is a surjection ABA \twoheadrightarrow B, then there is an injection BAB \hookrightarrow A.
  • Axiom of Choice (AC\mathsf{AC}): every family of nonempty sets has a choice function.

Known Partial Results

  • PP\mathsf{PP} is known to imply several weaker choice-like consequences.
  • PP\mathsf{PP} has been shown equivalent to AC\mathsf{AC} under various additional hypotheses, but no such additional hypothesis is known to be eliminable in general.
  • The problem has resisted both a proof of the implication and the construction of a permutation/symmetric or forcing model separating PP\mathsf{PP} from AC\mathsf{AC}.

Notes

This problem is notable for its longevity — it has remained open for over a century despite being one of the most natural questions one can ask about weak forms of choice, and despite extensive work relating PP\mathsf{PP} to essentially every other known choice principle.

Additional References

  • Rubin, H. and Rubin, J. E. "Equivalents of the Axiom of Choice II." North-Holland, 1985 (catalogs known implications among choice principles, including this one).
  • Halbeisen, L. "Combinatorial Set Theory: With a Gentle Introduction to Forcing." Springer, 2012 (survey of choice principles, including PP\mathsf{PP}).

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