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7dEdited

Some footnotes:

1) Although a lot of online sources make that error, it's important to underscore that infinite cardinals such as ℵ₀ are *not* set sizes in any plain meaning of this term. The whole point is that there are several ways of measuring set size that are equivalent for finite sets, but *completely diverge* once you start talking about infinite sets. Cardinals are just one of the resulting hierarchies; there are other approaches that give you different results. Many internet attempts to "debunk" Georg Cantor's work are based on this misunderstanding.

2) In the same vein, you often hear about "cardinal numbers", but it's best not to think of infinite cardinals as normal (ordinal-like) numbers. Compared to ordinals, they're different mathematical beasts. They are not a label for a specific set; they are a label for an entire class of sets. There's a lot of baggage that comes with that.

3) There are multiple infinite ordinals that will have the same cardinality. For example, ℵ₀ (the cardinality of ℕ) covers ω, ω + 1, ω · 2, and so forth; ω is the "smallest" ordinal in the ℵ₀ class.

4) There are ordinals of higher cardinality, too. If you take the set of all the first-order "counting number" ordinals that we know how to construct (0, 1, 2, ..., ω, ω + 1, ..., ω·2, ..., ω·3, ...), you will get a structure that embeds infinitely many infinite sequences separated by successor-discontinuities (see article). This construction corresponds to higher cardinality that can no longer be mapped to natural numbers. It nets us a new ordinal, ω₁, and a new cardinal ℵ₁. We can't prove or disprove that ℵ₁ is the same as the cardinality of ℝ.

5) If you read https://lcamtuf.substack.com/p/09999-1, you might be wondering if the arithmetic of infinite hyperintegers is different from infinite ordinals. It is, essentially because of the addition of some extra rules. That said, the basic meaning of ω is the same.

6) Toward the end of the article, we introduce the set of real numbers. The construction of reals is left as an exercise for the reader.

7) In addition to folks who object to the concept of infinity, there is a small number of mathematicians and philosophers who dislike set theory. For example, one prominent philosopher believes it's nonsensical to make a distinction between x and a set of containing x: https://ontology.buffalo.edu/04/AgainstSetTheory.pdf

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HN endorsements, for posterity:

"Why read this? Why be exposed to this slop?"

"A watered down Intro to Mathematics 101"

If I could read these comments, I would be very upset

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