I know that a similar question has been asked regarding finitism, but I'm interested in ultafinitism. That is, we define a set of numbers that has a specific upper limit. For argument's sake - let's say there are only 2 numbers: 0 and 1. So 1+1 is undefined because there is no number 2...

Does Gödel incompleteness theorem hold for these numbers? What about if the upper limit is higher - 100 or a googol?

So the question then is twofold - first, would an arithmetic limited to a sufficiently small subset of natural numbers be complete. And second, if so how high can we make that limit before it becomes incomplete?

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    $\begingroup$ I think the negative reception to this question is somewhat excessive, and have upvoted to compensate. $\endgroup$ Feb 21 '20 at 17:07
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    $\begingroup$ I don't think that a specific upper limit is the right way to think about ultrafinitism. All ultrafinitists believe in two, because everybody can count that high; it's a googol that they doubt, because nobody can count that high. Still, ultrafinitists generally accept that, whenever $n$ exists, so does $n+1$. (In theory, you can prove that a googol exists using only this axiom, step by step. But in practice, this proof has about a googol steps, and you're not physically capable of producing it.) $\endgroup$ Feb 22 '20 at 2:42

I'm not really sure what you're asking. Certainly for any natural number $n$, the structure $\mathbb{N}_n$ consisting of the natural numbers up to $n$ (with $+$ and $\cdot$ interpreted relationally in order to be a legitimate structure) is trivially decidable, so in that sense Godel doesn't apply to it(s theory).

But there's a huge issue here: checking whether a sentence of length $<n$ is true in $\mathbb{N}_n$ requires more than $n$ steps in general. So the decidability of $\mathbb{N}_n$ is not satisfying from an ultrafinitist standpoint, since the completeness itself isn't "justifiable within $\mathbb{N}_n$." Meanwhile, the non-ultrafinitist won't be impressed either since $\mathbb{N}_n$ is too limited a structure to treat even basic arithmetic. So this dodge doesn't seem satisfying from any perspective.

It's worth noting that Godel's second incompleteness theorem can be avoided non-trivially with a trick of arguably ultrafinitist spirit: there are theories which interpret a decent amount of arithmetic but which prove their own consistency, and a key component of such theories is that they do not prove that multiplication is total, so in a weak sense have ultrafinitistic flavor. These were first studied by Dan Willard; see here.

However, Godel's first incompleteness theorem still applies to these. Indeed, no theory which can prove each true quantifier-free sentence about $\mathbb{N}$ and, for each $k\in\mathbb{N}$, can prove $$Init_k:\quad\forall x(\underline{k}<x\vee\bigvee_{0\le i\le k}x=\underline{i})$$ (where "$\underline{m}$" denotes the numeral corresponding to $m$) is going to be complete, consistent, and computably axiomatizable (see e.g this paper of Ritter). Note that such theories are not required to prove that multiplication is total, or that $<$ is a linear ordering of the universe, or so on: they are truly quite weak.


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