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I asked a question on multiset.
If and , is ?
I answered you over at multiset. It's .
I asked another question at multiset.
I have added my thoughts to the discussion.
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<p>I added a question to <a href="http://ncatlab.org/nlab/show/multiset">multiset</a></p>
<blockquote>What would a colimit over an MSet-valued functor <img src="https://nforum.ncatlab.org/extensions//vLaTeX/cache/latex_1dac663271b525f37d22d6ed48334c38.png" title="F:A\to MSet" style="vertical-align: -20%;" class="tex" alt="F:A\to MSet"/> look like?</blockquote>
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I proposed an alternative definition for the intersection of multisets, but would be interesting in hearing arguments for and (more likely) against.
Thanks Toby for making me look at a simple example to see the depth of nonsense I was spewing about intersections :)
I think I am happy with it now, but while convincing myself I came back to an issue that has bothered me. On multiset, I think you and I went back and forth on whether we should allow (I removed "nonzero" from "nonzero cardinal numbers" and I think you put it back). In those papers by Syropolous, he allows and I kind of think we should as well.
If we were to allow , then I would define
This only makes sense if we allow .
I just doodled a proof that with this definition of set difference we have
What is the reason for requiring the cardinal numbers to be nonzero? Can we change it to allow zero?
PS: Thanks for removing the query box. I was going to remove it after I was sure you saw it, but only now remembered I could have removed it and told you about the change here.
In the simple example you gave, we have
and
so that
With the above definition of set difference (which requires allowing ) we'd have
and
Note that
so that
The reason for restricting attention to nonzero cardinal numbers is to make it unambiguous what is the underlying set of the multiset. If you remove ‘nonzero’ from the definition on the page, then and become different, even though they should both be (whatever that is).
Really, this comes from trying to make a material theory of multisets to match a material theory of sets. You get a clean theory by doing one of the following:
All of that stuff about intersections and inner products and so forth can only refer to (1). So we might as well allow as a multiplicity, but then we should do everything relative to . But all of the stuff about morphisms of multisets is about (2).
It's probably more confusing than it's worth to try to mix (1) and (2). I'd only ever seen multisets used for (1) until I saw those references about morphisms of multisets on the Café. But if you come from a material (ZFC-like) conception of sets, then it's very natural to mix them, which means that you need to keep adjusting the underlying set of a multiset, to keep it appropriate for (2), whenever you do an operation appropriate for (1). And that's what we've been doing on multiset.
And you can do this! You can define to have the right underlying set such that takes only nonzero values; I just mentioned how at multiset.
Thanks. So what would you suggest we do? Mix (1) and (2) or choose one? For the nLab, (2) almost seems for appropriate.
I'm not sure which one I'm using when I write . Is it closer to (2) with and ? I was trying to shy away from introducing a universal set.
What is the best notation? I can rewrite the examples once a notation is settled.
Sometimes I'm even tempted to treat multisets as vectors and write things like
By the way, did you see my extremely speculative Colimit as a Linear Regression of Multisets?
Edit: Oh, by the way, I moved the "Discussion" to the bottom because "Examples" and "References" were buried at the bottom of the page. I usually think of "Discussion" as being an addendum that appears at the end while "Examples" and "References" are part of the main content. No big deal. I really just wanted the Example to appear before the long discussion and took the References along for the ride.
So what would you suggest we do?
I think:
Note the difference between (1) and (2), explain why and how one might conflate them, then cover things separately. Possibly this means eventually branching to separate pages.
I'm running of time to be online now, but I can do this later.
I started some notes at ericforgy:Multiset. Feedback welcome!
When I was writing the section "How can we settle this?" at ericforgy:Multiset, I got a sense of deja vu. I've only begun reading Goldblatt's very cool book, but I was wondering if there was some way we can interpret the cardinality
as a subobject classifier for some topos?
Instead of Set, maybe some topos in a category of elements of some universe ? Or something...
Edit:
I guess another similar question is can we think of Set as a topos on a category of elements with subobject classifier
Since objects in the category of elements are, well, "elements" then I was thinking the subobject classifier tells you if the element is in the set .
Then
tells you not only whether the element is in the multiset, but also tells you how many times it appears.
<p>I would love to have a copy of this paper:</p>
<p><a href="http://cat.inist.fr/?aModele=afficheN&cpsidt=13517583">The category of M-Sets</a></p>
<p>Abstract. A topos is a category which looks and behaves very much like the category of sets, and so it may be thought of as a universe for mathematical discourses. One of the very useful topoi in many branches of mathematics as well as in computer sciences is the topos MSet, of sets with an action of a monoid M on them. It is well known that MSet, being isomorphic to the functor category SetM, is a topos. Here, we explicitly give the ingredients of a topos in MSet and investigate their properties for the working scientists and computer scientists. Among other things, we give some equivalent conditions, such as the left Ore condition, to <img src="https://nforum.ncatlab.org/extensions//vLaTeX/cache/latex_e114a9b563636b5006b7f2b0e27a5e45.png" title="\Omega" style="vertical-align:-20%;" class="tex" alt="\Omega" />, the subobject classifier of MSet, being a Stone algera. Also the free and the cofree objects, as well as, limits and colimits are discussed in MSet.</p>
It sounds like a lot of that paper would be in the way of a very good exercise for someone who is starting to learn topos theory. It might even be a good exercise to carry out here on the Lab: learning to do topos calculations just with one's bare hands and brain.
I would start a "journal club"-like page if I could somehow manage to get my hands on the paper (hint) :)
Edit: Argh. I just realized I packed away my copy of Goldblatt and it will be inaccessible for a few weeks.
We don't even need the paper or Goldblatt; we can get started right now without them. For example, what is M-Set exactly, and what does the author mean by saying that M-Set is isomorphic to Set^M? Then: how does one calculate finite products in M-Set/Set^M? (There's a good chance you know this already.)
We can go on from there to discuss general limits and colimits in M-Set, cartesian closure, the subobject classifier, and so on. This could be fun and a very good learning experience, and there are people right here who know this material rather well.
I have a vague enough understanding of multisets (which I hope are related to M-sets) to be able to perform some computations, but I'm still struggling with the right way to think of them. You can get a feel for what I know and don't know from
where I am trying to get a handle on "what are multisets exactly". I don't know :)
(feedback welcome!)
You can also see some confusion (on my part at least) of what a morphism should be between multisets at multiset.
In the meantime, I created a stub for M-Set where we can put some definitions and stuff and started a page
Oh no! According to Todd's comment on Understanding M-Set, it seems multisets may not be objects in M-Set.
I get distracted easily enough already. What I really want to understand is the category of multisets. In the references at multiset, they denote the category of multisets "MSet". Oh no!
Ah well. I responded to what you wrote at Understanding M-Set at any rate.
Thanks!
I would really LOVE to go through all this stuff. Especially while I have your attention! :)
I just wish we could find a category closer to my heart to do all those exercises. I wouldn't even mind doing an "Understanding Set" :) That might actually be a really good idea.
Maybe M-Set is close to my heart and I just don't realize it yet. Almost everything I'm tinkering with these days is related to multisets though (with a vague hope that they will help me with a speculative idea of mine
I thought that multisets were an example, but I am not sure anymore. If they are not, then I'd like to reserve M-Set or MSet for the category of multisets if possible.
Oh. Ok. We should probably call the category of multisets something else then to avoid confusion. Instead of MSet (with no space), which is too close to M-Set, how about "MultiSet" or "Bags" as Syropoulos calls it?
I like MultiSet or Multiset (if we can agree on what the morphisms are). Bags I like less, but that's just me.
Cool. I'll wait to create MultiSet until the morphism issue is settled.
Any ideas on the best n-Lab-esque way to define the category of multisets? Arrow theoretically somehow?
I already created it, at MSet, so I'll just move it now.
But yeah, there are at least two interesting ways to define the morphisms.
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