I am beginning to understand that we need to define an isomorphism class
for every extensible structure. But I think furthermore, that this could
be done in a generalized way:

isoClass= ( s e. _V |-> ...? )

The corrected definition of genCount would then look like:

genCount = ( s e. _V |-> ( n e. NN |-> ( # ` ( { e e. s | ( od ` e ) = n
} /. ( isoClass ` s ) ) ) ) )

Am 02.12.2024 um 21:46 schrieb Steven Nguyen:

reverse-chronological
(1) That's a great idea to generalize it. Though, for instance, rings
being group-isomorphic does not imply the rings are ring-isomorphic:
isrhm - Metamath Proof Explorer
<https://us.metamath.org/mpeuni/isrhm.html> so it's imagine it's
somewhat limited. I'm not very familiar with category theory so maybe
I'm missing something.

(2) genCount (non-generalized) is another kind of
generalization, again great idea. And corresponding to (1), the
particular definition using ~=g could be named grpCount,
genGrpCount, or #_grp (whichever feels most familiar)

(3) As for prime numbers we have df-ppi - Metamath Proof Explorer
<https://us.metamath.org/mpeuni/df-ppi.html> and we can verify what
the nth prime number is by, say, ppi3 - Metamath Proof Explorer
<https://us.metamath.org/mpeuni/ppi3.html>

On Mon, Dec 2, 2024 at 11:23 AM Peter Dolland <[email protected]>
wrote:

    Steven, what do you think about

    genCount = ( s e. _V |-> ( n e. NN |-> ( #  ( { e e. s | ( od  e )
    = n } /. ~=g ) ) ) )

    ? So, we would have

    a1 = ( genCount ` Grp )
    a688 = ( genCount ` Abel )
    a122 = ( genCount ` Poset )

    etc.

    Am 27.11.2024 um 10:31 schrieb 'Peter Dolland' via Metamath:

    Thank you, Steven, for formalizing oeis-a1! I think, there will
    be a general counting function assigning to any extensible
    structure class 𝒞 and a given cardinality ק the cardinality of
    the quotient of the subclass of 𝒞 having carrier sets with
    cardinality ק . The restriction of this function to finite
    cardinalities will give the OEIS sequences.

    But first I have a few questions about using Metamath and set.mm
    <http://set.mm>:

    (1) Is set.mm <http://set.mm> (without proofs) available as pdf?
    The book describes how I can create LaTeX input for individual
    statements or proofs, but not how the entire structure, including
    the important introductory paragraphs, can be integrated. Cross
    references to definitions would be nice.

    (2) Either is there another tool to get the definition of a
    symbol in a quick way? (also vim macros are welcome!)

    (3) I would like to test your definition of a1. But I do not
    understand, how to do this without changing set.mm
    <http://set.mm>. There is no second read possible.

    These are not the only questions I have when dealing with
    Metamath. But I hope that answering them will make it a little
    easier for me to get started.

    Am 23.11.2024 um 19:51 schrieb Steven Nguyen:
    As a current description, by default people work on their own
    often-overlapping
    topics, with the help of the community answering any questions.
    I'm unsure so
    I'll leave the question of contributors open.

    The project proposed here seems highly interesting. I think the
    main part
    of the work is simply defining all the functions, since many
    concepts haven't
    been defined yet.

    One potential difficulty is that Metamath currently does not
    have much tooling.
    So computer-generated proofs are ironically tedious and manual
    to prove by
    default (not sure if https://us.metamath.org/mpeuni/631prm.html
    was done
    manually; https://us.metamath.org/mpeuni/ax-bgbltosilva.html is
    also illustrative).
    Computer-generated proofs are rare in math, luckily. But if the
    plan is to prove a
    very large amount of sequences then I imagine it would be more
    efficient to make
    the tooling for it first. Or (dun dun dun?) ask the Lean
    community. Teamwork
    makes the dream work, in this case the sequences are
    math-community-wide so
    I'd imagine the larger amount of people doing Lean proofs would
    make it easier.

    I don't see any other strategy than starting with some sequence,
    and start seeing
    what needs to be done (a lot). As a start, here's A1, informally
    the
    (number of groups of order N), the definition is
    (number of groups of order N, mod equivalence)
    a1 = ( n e. NN |-> ( # ` ( { g e. Grp | ( od ` g ) = n } /. ~=g
    ) ) )
    On Thursday, November 21, 2024 at 10:54:30 AM UTC-6 Peter
    Dolland wrote:

        Hello Metamath Community,

        I'm still new here. So let me explain my intention, what I
        want here. My
        original motivation comes from combinatorics. I am
        interested in the
        relationship between the specification of an algebraic
        structure (such
        as directed or undirected graph, magma resp. groupoid,
        semigroup, group,
        topological space, partition, etc.) and the sequence that
        indicates how
        many non-isomorphic instances of the structure there are
        over a set with
        n elements, where n=0,1,2,3,.... . These sequences are
        published on
        oeis.org <http://oeis.org>: A273, A88, A1329, A27851, A1,
        A1930, A41,... . For some of
        them, a generating algorithm (in some programming language)
        is also
        given. However, in none of these sequences is a formal
        specification of
        the underlying algebraic structure given, although this is
        often much
        simpler than the generating algorithm.

        My impression so far is that the desired specifications can
        be created
        with MetaMath without any effort, as long as they are not
        already
        contained in set.mm <http://set.mm>. A real challenge would
        probably be to verify the
        specified algorithms against the specifications. It seems to
        me that a
        basis of theorems would have to be created first. In
        particular, Polya's
        counting theorem would need to be formalised. But perhaps
        one of you
        knows what already exists on this topic and can be used.

        Irrespective of this, it also seems worthwhile to me to
        create a bridge
        between MetaMath and OEIS in such a way that MetaMath
        specifications are
        added to corresponding sequences. The aim here could be to
        improve the
        findability of sequences. For example, the additional
        criterion that the
        elements of the carrier set are to be regarded as different
        (labelled)
        could be achieved by adding an independent complete order
        relation. For
        example, compare the sequences A88 with A53763, A273 with
        A2416, A41
        with A110 or A1930 with A798. So you can see the first
        sequence is for
        each pair the unlabelled version. And the second ones yield
        the labelled
        version.

        I see a need for action that goes far beyond my areas of
        different
        expertises. The question now arises as to whether there are
        people in
        your community who are willing and able to support me in my
        search of a
        bridge as mentioned above. I have to admit that I have not
        yet delved
        very deeply into MetaMath and set.mm <http://set.mm>. If I
        find someone here, I really
        will have chance in order to acquire the necessary knowledge
        and skills.
        I am hopefully very convinced that MetaMath makes a decisive
        contribution to the further development of mathematics and
        computer
        science by formalising important statements.

        With best wishes,
        Peter Dolland

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