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    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeSep 28th 2012

    I looked at real number and thought I could maybe try to improve the way the Idea section flows. Now it reads as follows:

    A real number is something that may be approximated by rational numbers. Equipped with the operations of addition and multiplication induced from the rational numbers, real numbers form a number field, denoted \mathbb{R}. The underlying set is the completion of the ordered field \mathbb{Q} of rational numbers: the result of adjoining to \mathbb{Q} suprema for every bounded subset with respect to the natural ordering of rational numbers.

    The set of real numbers also carries naturally the structure of a topological space and as such \mathbb{R} is called the real line also known as the continuum. Equipped with both the topology and the field structure, \mathbb{R} is a topological field and as such is the uniform completion of \mathbb{Q} equipped with the absolute value metric.

    Together with its cartesian products – the Cartesian spaces n\mathbb{R}^n for natural numbers nn \in \mathbb{N} – the real line \mathbb{R} is a standard formalization of the idea of continuous space. The more general concept of (smooth) manifold is modeled on these Cartesian spaces. These, in turnm are standard models for the notion of space in particular in physics (see spacetime), or at least in classical physics. See at geometry of physics for more on this.

    • CommentRowNumber2.
    • CommentAuthorjin
    • CommentTimeSep 11th 2020

    Provide sketch of the content of the paper.

    diff, v49, current

    • CommentRowNumber3.
    • CommentAuthorDavidRoberts
    • CommentTimeSep 11th 2020

    Moved the Pavlovic–Vaughan reference to the reference section, cited it, gave link to the published version, and put proper TeX syntax on the additions.

    But I’m not sure of this sentence:

    \ldots the result is simply the stream of \mathbb{N}: ×××\mathbb{N}\times \mathbb{N}\times \mathbb{N}\times \ldots, which obviously can be identified with the real number set.

    Really? Is it so obvious that this infinite product can be so identified?

    diff, v50, current

    • CommentRowNumber4.
    • CommentAuthorMike Shulman
    • CommentTimeSep 11th 2020

    Looks more like Baire space to me.

    • CommentRowNumber5.
    • CommentAuthorDavidRoberts
    • CommentTimeSep 12th 2020

    I agree.

    • CommentRowNumber6.
    • CommentAuthorTodd_Trimble
    • CommentTimeDec 21st 2020

    The confusion with Baire space, noted in previous comments, has been rectified.

    diff, v51, current

    • CommentRowNumber7.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 21st 2020

    Changed the description as a locale a little. It was ambiguous in the previous version what the frame actually was, due to a plural/singular mismatch across two sentences.

    diff, v52, current

    • CommentRowNumber8.
    • CommentAuthorOscar_Cunningham
    • CommentTimeDec 22nd 2020
    • (edited Dec 22nd 2020)

    I changed the identification between ω×ω×\omega\times\omega\times\dots and +\mathbb{R}^+ to a nice one from Continued fractions and order-preserving homeomorphism that doesn’t require offsetting ω\omega and +\mathbb{R}^+.

    I also corrected an error that said that α\alpha and β\beta were both monotonic. The function β\beta is not monotonic, but is monotonic when restricted to a domain where α\alpha is constant.

    diff, v53, current

    • CommentRowNumber9.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 22nd 2020

    Clarified that it is the underlying sets of Baire space and the real numbers that are isomorphic, since Schroeder-Bernstein is being invoked, and that is false in the category of topological spaces.

    diff, v54, current

    • CommentRowNumber10.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 22nd 2020

    Crossed-linked stream in the terminal coalgebra proof.

    diff, v54, current

    • CommentRowNumber11.
    • CommentAuthorTodd_Trimble
    • CommentTimeDec 22nd 2020
    • (edited Dec 22nd 2020)

    Oscar, you’re right; thanks.

    • CommentRowNumber12.
    • CommentAuthorTodd_Trimble
    • CommentTimeDec 22nd 2020
    • (edited Dec 22nd 2020)

    However, I still like my original description, which works if we simply change from the product poset ω× +\omega \times \mathbb{R}_+ to the lexicographic one. We can keep yours, but I want mine reentered into the record.

    Added: are you sure β\beta is monotonic? I get β(1.9)>β(2.1)\beta(1.9) \gt \beta(2.1). I think in fact it’s the very same issue as before.

  1. I don’t understand what you mean about product vs lexicographic. If you go ahead and make the change I’ll look at what you wrote.

    • CommentRowNumber14.
    • CommentAuthorTodd_Trimble
    • CommentTimeDec 22nd 2020

    I’ll say it here first: if xyx \leq y, then (α(x),β(x))(α(y),β(y))(\alpha(x), \beta(x)) \leq (\alpha(y), \beta(y)) using the lexicographic order.

  2. Right. I think the confusion was that you thought that my change to β\beta was to make it monotonic, whereas in fact I simply changed β\beta to allow the use of ω\omega and +\mathbb{R}^+ directly rather than having to replace them with 2\mathbb{N}_{\geq 2} and [1,)[1,\infty).

    Then I unrelatedly changed the sentence that said that α\alpha and β\beta are monotonic to one which says that (α,β)(\alpha,\beta) is monotonic. And since we already mentioned in the definition of F 1F_1 that the order on ω×X\omega\times X is the ordinal product (which is lexicographic), I think the current wording already says exactly what you want.

    • CommentRowNumber16.
    • CommentAuthorTodd_Trimble
    • CommentTimeDec 22nd 2020

    I have added the earlier description, with the necessary adjustment. Please do not modify this addition without prior discussion.

    diff, v56, current

  3. You should remove my version then. I thought it was simpler, but it’s so similar to the other one that there’s no real gain in having both. If we want a second example there’s the function that sends a nonnegative real to the list of how many 11s there are between each 00 in its binary expansion.

    • CommentRowNumber18.
    • CommentAuthorTodd_Trimble
    • CommentTimeJun 15th 2021

    I tried to repair some wrong claims about terminal archimedean structures.

    diff, v67, current

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