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There seems to be some literature stating that Mahlo Universe[1][2] is the counterpart of Mahlo Cardinal in type theory, but I don't fully understand this point of knowledge.

More explicitly, I would like to know what condition is missing between CIC + a Mahlo Universe and IZF + a Mahlo Cardinal?

[1] Setzer, A. (2000). Extending Martin-Löf type theory by one Mahlo-universe. Archive for Mathematical Logic, 39(3), 155-181.

[2] Kahle, R., & Setzer, A. (2010). An extended predicative definition of the Mahlo universe. Ways of Proof Theory. De Gruyter. doi, 10(9783110324907), 315.

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  • $\begingroup$ Can you define (and give references for the definitions) the Mahlo Universe? $\endgroup$
    – J..y B..y
    Sep 12, 2022 at 10:49
  • $\begingroup$ @J..yB..y I'm talking about Anton Setzer's Mahlo Universe. References have been added. $\endgroup$ Sep 12, 2022 at 11:57
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    $\begingroup$ Already a bit painful is the fact that the CIC <-> IZF relationship is not entirely trivial. I think things get easier when considering CIC + EM vs ZF. $\endgroup$
    – cody
    Sep 12, 2022 at 17:26
  • $\begingroup$ @cody I would welcome this if it would alleviate your distress. The key to this question is the combination of the universe and the cardinal pair, not CIC and IZF. $\endgroup$ Sep 12, 2022 at 22:39
  • $\begingroup$ I'm not entirely sure that the Mahlo-ness washes out the difference in foundation (maybe it does). What result would you like/expect to prove? $\endgroup$
    – cody
    Sep 14, 2022 at 18:27

1 Answer 1

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It's awkward to ask yourself questions, but...

Rathjen, M. (2003). Realizing Mahlo set theory in type theory. Archive for Mathematical Logic, 42(1), 89-101.

The chapter 5, "Realizing set theory in Mahlo type theory" is the required construction for CZF + Mahlo Cardinal. The previous section shows why this construction does satisfy the definition of Mahlo Cardinal.

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  • $\begingroup$ Neat! Ideally detailing the construction in a few words here would be really useful for future reference. $\endgroup$
    – cody
    Feb 25, 2023 at 4:28

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