## Archive for August, 2013

### Determinacy of Borel games II

August 31, 2013

By the end of the previous post, I had said what a Borel subset of $\mathbb{N}^{\mathbb{N}}$ was, and what a determined subset was. Martin’s theorem is the statement that all Borel sets are determined.

I also commented that an intersection of two determined sets does not have to be determined, which suggests that in order to prove that all Borel sets are determined, we will need to find a clever inductive hypothesis. This hypothesis should be of the form, “All Borel sets of index less than $\alpha$ have property P,” where having property P implies that you are determined, and it is also preserved when you take countable intersections and unions.

Since the property of being determined is quite a strange property, it seems rather unlikely that we will be able to find a much simpler property that does the job. So it is natural to look for a property that is itself related to games and determinacy. But what might such a property be?
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