Explosion use in math

[Patrik Eklund]

And it's maybe trivial to point this out, but we always need to be 
careful about not identifying ø with {ø}, with {{ø}}.

Somebody recently asked about monads. Studying the powerset and term 
monads are useful. For the powerset functor P there is the 
multiplication of the powerset monad that brings PP back P, but if we 
study the double powerset PP we have to be care as PP can be composed by 
the covariant P with itself or by the contravariant P with itself. Both 
PPs are covariant but with different properties. For instance, the 
filter monad is a submonad of the contra-contra PP but not of the coco 
PP. Then the Eilenberg category of the filter monad is interesting also 
as it it becomes to be isomorphic with the category of compact 
topologies. Understanding these underlying phenomena is also useful.

As for the term monad we often say it is idempotent TT=T since a "term 
of terms is a term", but when we really examine that T as a monad we 
realize it's not an equality TT=T but it is equal "up to isomorphisms".

Studying compositions like PT (sets of terms) is interesting as this 
brings in the "swapper" transformation t : TP -> PT and Beck's 
distributive laws. Again, studying such things will show how PT is 
doable, but TP as a monad not, since a swapper s : PT -> TP does not 
exists. A similar doable and not doable is in the case of P(X x X) and 
PX x PX.

Finally, for the purpose of this e-mail, if we bring in many-sortedness 
into T, the extension from one-sortedness to many-sortedness is not 
trivial. Doing it with sets enables to bend the curves, but when you 
construct that many-sorted term monad categorically, you will see how 
subtle it is.

---

So indeed, be careful with the emptyset and the brackets around it, and 
Vaughan is right on target on "the purpose of illustrating that category 
theorists view sets differently from how set theorists view them".

Somebody also pointed out earlier how some categorists prefer to go 
morphism before going object, where "singleton" is a property of a 
morphism, not a feature of the object. And, as Vaughan says, there is 
one empty set, but many singletons. And clearly, depending on the 
category, this situation is different. In Set, the empty set is the 
unique initial object and singletons are terminal objects, but in many 
other categories it looks different. This connection with terminal and 
initial again shows how categorists like morphisms to precede objects.

---

Best,

Patrik

On 2022-05-26 00:30, Vaughan Pratt wrote:

> Harvey Friedman offered the following as simple examples of 
> propositions supposedly most easily proved using "explosion" (reductio 
> ad absurdum).
> 
> The emptyset is included in every set.
> There is one and only one set that is included in every set.
> 
> While contemplating his claim (for the first I would have avoided the 
> use of explosion by arguing that "for all x in emptyset, x is in Y" 
> holds vacuously) it occurred to me that the second is only true of the 
> category Set up to isomorphism, but that in that sense of "only one" 
> there is also only one singleton in Set, one doubleton, and so on.  In 
> set theory however there really is only one empty set, but many 
> singletons etc.
> 
> So Harvey's example serves not only his original purpose but also the 
> purpose of illustrating that category theorists view sets differently 
> from how set theorists view them.
> 
> Vaughan Pratt
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