A categorical approach to forcing?

When doing set theory we tend to take pride in the fact that it’s something unique, something that is doing things in a very different way from the rest of mathematics. Take things like transitive sets, forcing extensions, elementary embeddings as well as syntactic considerations in results like Shoenfield absoluteness. But there are times when we might be better off by borrowing terminology, and maybe even results, from other fields. In this post I will just give a potential example of such a thing: namely, considering the structural side of forcing from a categorical point of view.

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A universe of hierarchies

When doing set theory (or mathematics in general) we’re working inside some universe, usually denoted by V. Since we can’t work with everything there is (in a first-order way), we resort to working with initial segments of V. The confusion then arises, since what do we mean by an initial segment? Some prefer to work with the “rank-hierarchy” V_\alpha and others prefer to work with the “hereditary cardinality hierarchy” H_\kappa. It gets even worse if we’re working in Gödel’s constructible universe L, since then we also got Gödel’s L_\alpha hierarchy and Jensen’s J_\alpha hierarchy. How do we picture these hierarchies? What are their relation to each other?

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Distribution

The notion of distributivity comes from the Latin word distribut-, meaning “divided up”, and has since evolved into how mathematics deals with things that are divided up. This starts back in school when we learn that a\cdot (b+c)=ab+ac. This property can be generalised in the language of Boolean algebras, still maintaining the intent of dealing with divided stuff, leading to the axiom of choice being a notion of distributivity as well!

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Mice and long games

When dealing with games in general, we can vary different parameters. We could vary (1) how big the payoff set is, (2) which objects we’re playing and (3) for how many rounds we’re playing. In a ZFC context, which is what I’ll be working with here as well, I’ve previously written about what limitations we’re facing. In particular, when we restrict ourselves to definable games then we can’t have determinacy of games on integers of length \omega_1+\omega. Restricting ourselves to definable games of countable length on the integers, what large cardinal strength do we obtain?

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A travel guide to the large cardinals

Looking at a map of the large cardinal hierarchy for the first time can be a dizzying experience. What are the differences between them, and which ones are similar? Some of them are defined using partition properties and some of them are defined using elementary embeddings, and others have a whole myriad of equivalent characterisations! What’s the intuition about the different sections of the hierarchy, and what type of set theorists are working in each section?

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Talks: Ramsey-like cardinals

I’m giving a contributed talk at the inner model theory conference in Girona and at the Set Theory Today conference in Vienna. Both talks are going to be on results from my recent paper, where the Girona talk will be more specialised in a game-theoretic direction and the Set Theory Today talk will be more of an overview of the Ramsey-like cardinals and some of our results. Here are a couple of abstracts.

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Encode/decode

There’s a very neat way of encoding any set as a set of ordinals, which has the somewhat peculiar feature of it being hard (which here meaning that it requires the axiom of choice) to encode sets, but easy to decode them. Like some kind of a very ineffective crypto-system.

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