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People like to talk about the "standard model" of Peano arithmetic - the "obvious" set of natural numbers you can define using set theory. But this depends on your choice of set theory! So let's study the T-standard model for any theory T. (1/n)https://diagonalargument.com/2019/09/14/non-standard-models-of-arithmetic-12/ …
I remember watching the animation of a sphere inversion a few years ago and somehow was even more confused
Yeah, it's confusing to me too. (When you know nothing about a subject you're at a local minimum of confusion, and you have to climb up before you can get back down. The mountain pass is littered with the bones of people who got stuck up there.)
I saw two sphere inversion videos; the newer one (AMS? MAA?) with narrators Xanthippe and Yorick was much easier to visualize and follow (based on a simpler proof using "corrugated" manifolds).
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