Large Countable Ordinals (Part 3)

Last time we saw why it’s devilishly hard to give names to large countable ordinals.

An obvious strategy is to make up a function f from ordinals to ordinals that grows really fast, so that f(x) is a lot bigger than the ordinal x indexing it. This is indeed a good idea. But something funny tends to happen! Eventually x catches up with f(x). In other words, you eventually hit a solution of

x = f(x)

This is called a fixed point of f. At this point, there’s no way to use f(x) as a name for x unless you already have a name...

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Published on July 06, 2016 18:00
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