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Comment by ChrisLomont | Hacker News Reader

> uniquely in finite space.

If this is your definition, it falls to the barber paradox.

In finite space, if we're this sloppy, I can define the simplest real number not definable in finite space.... But I did it in finite space.... This this notion needs to be made more precise ... Then the next one falls... And so on.

Go ahead, you seem to know enough math to know what constitutes a solid definition and a proof. Try to define the set you're talking about, making sure it's not circular nor excludes any numbers you think it should contain. It's not trivial, nor even perhaps possible.

This is what I mean by you're going to have a very hard time making your definition precise enough to be a true statement.

People have tried your path many, many times. From my poking at the lit on it it never works. Chaitin has spent decades on these things, and the result always falls just outside precise enough to make a proof.

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"The successor of the largest natural number that can be written in less than twenty English words" does not uniquely identify a number. The only thing that fact proves is that not every string of English words uniquely identifies a number. But it's a pretty weird proof of that, "watermelon" would suffice just as well.

The fact that not every finite string of symbols in every notation identifies a number uniquely doesn't break anything.

This is in fact just like the barber paradox. "I cut the hair of everyone in town who does not cut their own hair" is also not a paradox unless you add in assumptions not present in the original formulation. (Hint: where does the barber say they don't also cut the hair of people who do cut their own hair?) All you're doing is constructing straw men to be smarter than.

But you haven't actually addressed my main point: the only thing those numbers provide is making the limits Cauchy sequences exist. And that isn't actually useful outside of writing proofs that assume they do. For any other purpose, those numbers might as well not exist.

>All you're doing is constructing straw men to be smarter than.

No - for example, the barber paradox (in the form of Russell's paradox) is what led to a revolution in set theory around 1910 and Whitehead's Principia Mathematica. It is a real problem, and led to the notions of classes and sets being distinct (and universes? I forget..)

The sloppy "definable number" game runs afoul of exactly these issues. If you look on google scholar for "definable reals" or "non-constructive reals" you can get a taste of the research still being done in this area. It's not as simple as I think you think it is.

The short of it is that there are many, many notions of definable, and they are not equivalent. Thus I asked you to provide one you think is suitable for your claims.

> the only thing those numbers provide is making the limits Cauchy sequences exist.

This is far from true - depending on the (set-theoretic) model of the reals, you end up with all sorts of interesting pure math questions. If I recall, certain sequences of modules over the reals end up with different homology depending on crazy subtle questions of the model of reals.

There's also a lot of equivalent methods for defining reals, most require proof to show they are equivalent.

There are lot of interesting questions on the constructions of the reals (Conway called his surreal numbers his best discovery, and they are spectacular - they spit out the reals in a really interesting way from only two rules - no Cauchy sequences or Dedekind cuts or common constructions in sight. They were discovered analyzing perfect information two player games....). Here's a paper with many cool constructions of the reals [1]. Claiming the only thing they provide is Cauchy sequence limits is missing a lot of other useful properties.

So you may not find the reals useful, but they certainly are incredibly deep and fascinating. But they are useful far beyond the concept of Cauchy sequences.

[1] https://arxiv.org/pdf/1506.03467v1.pdf

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