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Iterating logarithms and well-foundedness

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Abstract · v1

We prove that there is no sequence {Xn}n∈ω\{ X_n \}_{n\inω} such that for all nn, there is an injection from P(Xn+1)\mathcal{P}(X_{n+1}) to XnX_n under a restricted set of axioms that conforms to the "working mathematician's" intuition (excluding Choice, Replacement, and Foundation). We also show such a sequence cannot be defined using a formula either. More precisely, it is inconsistent with Zermelo set theory minus Infinity to have a formula Φ(⋅,⋅)Φ(\cdot,\cdot) such that for each n∈ωn\inω there are XX and YY with Φ(n,X)Φ(n,X), Φ(n+1,Y)Φ(n+1,Y), and P(Y)\mathcal{P}(Y) being injectable into XX. We finally show that the theorem fails if we allow infinitely many definitions, one for each term of the sequence, by using Hamkins, Linetsky, and Reitz's pointwise definable extensions of countable models of ZFC.

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