Categorification of large cardinals?

When working with most of modern set theory we tend to transcend ZFC, always working with some strong background hypothesis, whether it being the existence of some elementary embedding, a colouring for some partition property, a generic for some uncountable poset or something completely different. When it comes to using these strong hypotheses in mainstream mathematics it seems that we hit a brick wall, as most of our strong hypotheses don’t easily translate to the language of everyday mathematics.

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Pcf scales and squares

The last few posts I’ve been covering a characterisation of pointclasses that admit scales. To make scale theory even more confusing there’s a completely different notion of scale, which really has nothing to do with our previous one — this one being of a more combinatorial nature. To avoid unnecessary confusion I’ll call these new objects pfc scales (but usually they’re simply called scales as well, however).

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Scales 101 – part III: moving to L(R)


The last two posts covered the ‘classical’ theory of scales, meaning the characterisation of the scaled pointclasses in the projective hierarchy. Noting that \bf\Sigma^1_n=\bf\Sigma_n^{J_0(\mathbb R)} and \bf\Pi^1_n=\bf\Pi_n^{J_0(\mathbb R)}, the natural generalisation of this characterisation is then to figure out which of the \bf\Sigma_n^{J_\alpha(\mathbb R)} and \bf\Pi_n^{J_\alpha(\mathbb R)} classes are scaled, for \alpha>0. This is exactly what Steel (’83) did, and I’ll sketch the results leading up to this characterisation in a couple of blog posts. This characterisation is also precisely what’s used in organising the induction in core model inductions up to \textsf{AD}^{L(\mathbb R)}.

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