Separation of choice principles in elementary topoi

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Let N\mathbb{N} denote the natural numbers, let 2={0,1}⊆N\mathbf{2}=\{0,1\}\subseteq\mathbb{N}, and let ACN-bounded\mathrm{AC}_{\mathbb{N}\text{-bounded}}, ACNN\mathrm{AC}_{\mathbb{N}\mathbb{N}}, and ACN2\mathrm{AC}_{\mathbb{N}\mathbf{2}} denote the choice principles defined in the paper. Let RC\mathbb{R}_C and RD\mathbb{R}_D be the Cauchy and Dedekind real objects, respectively.

Choice-separation conjecture.

  1. There exists an elementary topos in which ACN-bounded\mathrm{AC}_{\mathbb{N}\text{-bounded}} holds but ACNN\mathrm{AC}_{\mathbb{N}\mathbb{N}} does not.
  2. There exists an elementary topos in which ACN2\mathrm{AC}_{\mathbb{N}\mathbf{2}} holds but ACN-bounded\mathrm{AC}_{\mathbb{N}\text{-bounded}} does not.
  3. There exists an elementary topos in which ACN2\mathrm{AC}_{\mathbb{N}\mathbf{2}} holds but RC≠RD\mathbb{R}_C\neq\mathbb{R}_D.

These claims seek elementary-topos models separating bounded, natural-number-valued, and two-valued choice, and the third would imply the second. The supplied text provides no resolution.

References

Primary source

Martin Escardo and Alex Simpson, “Euclidean interval objects in categories with finite products”, arXiv:2504.21551 (2025).

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