Re: ​Re: Mathematics with the potential infinite

[Haim Gaifman]

You can certainly say if you assume potential infinity that there is no  greatest integer (because n+1 > n). 

Certain infinite sequences of natural numbers can be justified on the basis of potential infinity.  Roughly speaking, as long as you can define them without using quantification over  all natural numbers, but only over “sufficiently large but finite initial segments of the natural numbers”, you are OK. For example, the iterations of the exponential function: 1, 2, 4, 16, 2^(16), 2^((2^16)),… is legitiemate on the basis of potential infinity.

Thus, you can handle  dyadic rationals.

But, in general, to say that between an ascending sequence 
and a descending sequence there is a gap which can be filled exactly by one sequence, you must at least quantify  over all natural numbers. This is what one does in analysis when one defines the least upper bound and the greatest lower bound, and other standard notions. In "For every delta there is an epsilon” you can choose delta and epsilon to be of the form 1/m and 1/n, and in general this would involve quantification over all natural numbers.
  
Best, Haim Gaifman

> On Jan 29, 2023, at 12:05 AM, Vaughan Pratt <pratt at cs.stanford.edu> wrote:
> 
> Thanks, Haim, good to hear from you, and good to know you're watching FOM.
> 
> You seem to be answering my second question, "Can R be shown to be uncountable using only potential infinities" in the negative.  That was my intuition, as I can't imagine how Cantor's proof that R is uncountable could be carried out using only potential infinities, which was why I asked it.
> 
> But what about my first question?  If one can talk about integers as involving only potential infinities, surely one can do that with dyadic rationals (or general rationals for that matter, although the dyadic kind suffice her.0e).
> 
> Can one say that there is no greatest integer without going beyond potential infinities?  If so, why can't one speak of an ascending chain of dyadic rationales below a descending chain of dyadic rationals such that neither has a greatest (resp. least) element?  And the extra condition of at most one element between those two chains is surely finitary.
> 
> If the concept of "only potential infinities" is sufficiently well defined, it should be possible to see (i) precisely which condition has been violated in the above definition of a real, and (ii) whether some slight adjustment to that condition would overcome the violation.
> 
> Defining a real to be either a dyadic rational or the unique dyadic irrational filling a gap in my sense has elements in common with both Cantor's Cauchy sequences and Dedekind's cuts, but (potentially) without involving the actual infinities of either.
> 
> Moreover the language is that of Presburger arithmetic (multiplication does not enter into the above definition of a real) and therefore the sorts of undecidable questions that prompted Brouwer to introduce his notion of apartness should not plague this naive definition of a real number.
> 
> Vaughan Pratt
> 
> On Sat, Jan 28, 2023 at 7:03 PM Haim Gaifman <hg17 at columbia.edu <mailto:hg17 at columbia.edu>> wrote:
> Dear Vaughan,
> Long time no hear no see, and it is very nice to hear from you.
> The restriction of subscribing only to potential infinities (which can be traced back to Aristoteles) is Hilbert’s so called finitist position; Abraham Robinson agrees with him. Only structures based on proper initial segments of the natural numbers: {0, 1, 2,…, m} are accepted as legitimate, but for every m, if n > m, one accepts also the extension based on {0, 1, …., m, m+1,…., n}. The functions and/or relations that come with these structures are the usual functions and/or relations of PA (Peano Arithmetic). Of course, the functions are partial functions  , given the restrictions on the domain. 
> 
> PA, which is based on the standard model N of natural numbers, is much much… stronger than the theories
> that arise within the framework of potential infinity.
> One such interesting theory has been proposed by Skolem
> and is known as PRA for Primitive Recursive Arithmetic. 
> 
> Now your question, if I understand you correctly, asks for a way of describing an uncountable structure using only potential infinities.This would be impossible, unless you allow countable non-standard model for the theory linearly ordered groups.
> 
> Best, Haim Gaifman
> 
>> On Jan 28, 2023, at 3:13 AM, Vaughan Pratt <pratt at cs.stanford.edu <mailto:pratt at cs.stanford.edu>> wrote:
>> 
>> My apologies for not having previously followed threads on this topic.  However after seeing Stephen Simpson's message just now (Friday) it occurred to me to ask whether an uncountable set could be described using only potential infinities, for example the real numbers (R, *, 0, <=) as a linearly ordered group under addition, compatibly ordered in the sense that each of the group multiplication's two arguments is monotone: if x <= y then x*z <= y*z, and likewise for the right argument.  (* = +.)
>> 
>> Define a *geodesic* to be a nondegenerate linearly ordered group (G, *, 0, <=).  (Although G is not assumed abelian, the linear order makes it abelian.)  Examples include the integers, the dyadic rationals, every field between the rationals and the reals, and many non-Archimedean extensions thereof.
>> 
>> Call a geodesic G *gapless* when (i) it is dense, and (ii) for every nonempty suborder (U, <=) of (G, <=) having no least element, and every nonempty suborder (L, <=) of (G, <=) with L < U and having no greatest element, such that there is at most one element of G between L and U; then there exists an element of G between L and U.
>> 
>> I claim that every gapless geodesic is isomorphic to R with the above structure.
>> 
>> (Proof outline: Take any element x of G with 0 < x and pair 0 and x with 0 and 1 in R.  Pair the integers in R with the subgroup of G generated by x, cyclic and therefore abelian.  Repeatedly divide the intervals in (n, n+1) in G into two equal parts and pair the results with the dyadic rationals in (0,1), a dense set.  Pair each dyadic irrational q in R with the unique x given by the gaplessness condition for any L and U in G whose counterpart in R converges to q from each side.  Lastly, G must be Archimedean or there would be an empty gap between the finite and infinite elements of G.)
>> 
>> 1.  Do these definitions, claims, and constructions meet the criteria for only potential infinities?
>> 
>> 2.  Can R be shown to be uncountable using only potential infinities?
>> 
>> (Those familiar with Otto Hoelder's 1901 paper showing that every Archimedean linearly ordered group is isomorphic to some subgroup of R under addition, which may be anywhere between Z and R, may see some similarity of ideas in the above.)
>> 
>> Vaughan Pratt
> 

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