[FOM] 622: Adventures in Formalization 6

John Baldwin jbaldwin at uic.edu
Wed Oct 28 17:45:38 EDT 2015

On Wed, Oct 28, 2015 at 3:04 AM, Mitchell Spector <spector at alum.mit.edu>

> Addition on Conway's surreals, restricted to ordinals, is not the same as
> the usual addition on ordinals.  Addition of surreals is commutative, while
> the usual addition of ordinals is not commutative.
> In fact, addition of surreals, restricted to ordinals, is the same as the
> so-called natural addition of ordinals.  (I don't think I've ever seen
> natural addition of ordinals used in set theory.)
> Mitchell Spector

By the way, natural addition of ordinals is a useful tool for certain
ordinal ranks in model theory.
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