Definition talk:Relation

Just noted that relation on $S$ is defined under 'Relation on a Set' as well as under 'Endorelation'. --Lord_Farin 09:36, 20 January 2012 (EST)
 * N'est-ce pas? My view is that the "Endorelation" definition should stand (with its "also known as a relation on a set" kept), and the first instance of "Relation on a set" should be removed from here and put into Definition:Left-Total Relation instead with a similar "also known as", and to both of them a warning should be added that "relation on a set" has an alternative meaning, is ambiguous and therefore discouraged. --prime mover 10:26, 20 January 2012 (EST)
 * Agreed, that's best. Although my mind says that a relation on a set is an endorelation, maximal rigour and clarity should be achieved, so this is the only viable option. --Lord_Farin 10:47, 20 January 2012 (EST)
 * Job done. Does that work for you? --prime mover 13:34, 20 January 2012 (EST)
 * Sure. One down, hundreds to go. But we are undeterred. --Lord_Farin 18:20, 20 January 2012 (EST)

Question
Hmm, there seems to be a problem here. It doesn't make sense to ask if a set $R \subseteq X \times Y$ is left-total or right-total or neither, because neither $X$ nor $Y$ is uniquely determined by $R$. The only sensible thing is to ask if the ordered triple $\left({X, Y, R}\right)$ is "left-total" or "right-total." So either we're going to have to say that $\left({X, Y, R}\right)$ is the relation, or we're going to have to call $\left({X, Y, R}\right)$ a [name] and then say that the [name] is "left-total" or "right-total" or neither. --abcxyz (talk) 17:56, 20 October 2012 (UTC)


 * I think when someone asks if a relation is left-total or right-total the underlying set of the relational structure is implied. --Jshflynn (talk) 18:17, 20 October 2012 (UTC)


 * what he said --prime mover (talk) 18:18, 20 October 2012 (UTC)


 * So I guess you're saying it's something like Definition:Interior (Topology) where we say "interior" instead of the more specific "$\vartheta$-interior." Thanks; your comment just made me think of that. For some reason, it didn't occur to me. Fine, then. --abcxyz (talk) 18:30, 20 October 2012 (UTC)


 * I just realised I'm technically wrong because a relational structure only works for endorelations. For notationphiles have a look at Bourbaki's Theory of Sets Chapter 2 Section 3.1 to see how they ordered the triple that abcxyz describes differently :) --Jshflynn (talk) 18:28, 20 October 2012 (UTC)


 * The definition of which you refer, abcxyz, is already in this page at Definition:Relation, so we already have the required base covered. I can see where you are coming from: it can be argued that by just defining "relation" as just the subset of the cartesian product, it could be construed that the relation is just that. I suppose it is worth labouring the point - or would you suggest that the section alluded to (i.e. the one that treats a relation as a pair consisting of the product and the subset of that product) be made the main section?


 * Also note that the page Equality of Relations is explicit in its inclusion of the domain and range in the things it is defining equality on.


 * And the page defining left-total and right-total themselves include the domain and range in the discussion, by way of defining what they are. --prime mover (talk) 19:25, 20 October 2012 (UTC)


 * I just read all of this, and the first thing coming to my mind is: "It does make sense. The only problem is that we're not sure why." A similar reasoning applies to Talk:Open Set may not be Open Ball. We could of course write every thing with so much rigour that a proof checker would approve of it (with sufficient time input). However, this would inevitably sacrifice the readability and hence the usefulness of the site; I'd rather not go further down that road. Implicit stuff is good to be pointed out on definition pages, but critical is that it is also stressed that, when no risk of ambiguity (that's a very loose concept, of course - nonetheless) arises, we simplify notation for readability and brevity. --Lord_Farin (talk) 19:26, 20 October 2012 (UTC)


 * I don't think that defining a relation as $\mathcal R = \left({X, Y, R}\right)$, where $R \subseteq X \times Y$, is even close to unreadable. If we keep it as it is, I'm concerned that someone new to these concepts might look at Definition:Relation, Definition:Set Equality, and Equality of Relations and see them as inconsistent. Obviously, I prefer defining a relation as $\mathcal R = \left({X, Y, R}\right)$, but is it worth changing or does it not really add any clarity? --abcxyz (talk) 20:54, 20 October 2012 (UTC)


 * As I said, it's already covered in Definition:Relation. It's not unreadable, it's just unwieldy. --prime mover (talk) 22:16, 20 October 2012 (UTC)


 * Why write everything once? Lord_Farin I need you to implement a scroll bar on every page such that scrolling toward the right you have everything as explicit as metamath and toward the left you have it as terse and efficient as specialists write to one another. Oh and on top of that can you create some PHP script working with the regular expressions of Latex so that when I hover over some symbol it links me to the correct page? All by Monday please :) --Jshflynn (talk) 19:45, 20 October 2012 (UTC)

Fortunately, I have been working on that recently on my super secret PW clone. These are interesting ideas, though (but many specialists' notes would be no more than "Trivial", "straightforward", "obvious" and their consorts, of course. --Lord_Farin (talk) 19:55, 20 October 2012 (UTC)

Btw, would you be okay with the hovering thingie implemented in JavaScript (that's better suited to MathJax)? :) --Lord_Farin (talk) 19:57, 20 October 2012 (UTC)

On a more serious note, people interested can discuss stuff at http://mathim.com/ProofWiki. --Lord_Farin (talk) 19:59, 20 October 2012 (UTC)

As Ordered Triple
As the new section Definition:Relation is effectively a weaker implementation of Definition:Relation, and does not add anything, I wonder whether its presence is viable. Thoughts? --prime mover (talk) 22:52, 20 October 2012 (UTC)


 * I added it because the cases where $S = \varnothing$ or $T = \varnothing$ work out differently. --abcxyz (talk) 22:55, 20 October 2012 (UTC)


 * Fair enough. Now what we have to do is write a page explaining the differences between all three different approaches, and why it's important and what the consequences are of failing to take into account all these details. I'll leave it to you to fill in the details.


 * Also, a citation for your ordered triple approach would be nice. Unless you invented it, that is. --prime mover (talk) 23:08, 20 October 2012 (UTC)

Abcxyz I like your triple notation. See if you can find a much more interesting problem than this one.
 * $\left({X_1,Y_1,\mathcal R_1} \right) \left( {\supseteq, \subseteq, \subseteq} \right) \left({X_2,Y_2,\mathcal R_2} \right)$

(Pointwise) Implies:
 * $\left({X_1,Y_1,\mathcal R_1} \right) \left( {\cup, \cap, \leftarrow} \right) \left({X_2,Y_2,\mathcal R_2} \right) = \left({X_1,Y_1,\mathcal R_1} \right)$

Where $\left({X_1,Y_1,\mathcal R_1} \right) \left( {\cup, \cap, \leftarrow} \right) \left({X_2,Y_2,\mathcal R_2} \right)$ just means $\left({X_1 \cup X_2,Y_1 \cap Y_2,\mathcal R_1 \cap \left( {\left({X_1 \cup X_2} \right) \times \left({Y_1 \cap Y_2} \right)} \right)} \right)$. --Jshflynn (talk) 21:12, 22 October 2012 (UTC)


 * I don't understand what you mean. What's the problem? Could you please explain? --abcxyz (talk) 18:58, 23 October 2012 (UTC)


 * Give it no further thought abcxyz. I don't think I was thinking straight at the time. --Jshflynn (talk) 17:46, 31 October 2012 (UTC)

Terminology
We currently call a subset of $S \times T$ a "relation on $S \times T$". That is, we actually define "relation on" to mean "subset of", but only when applied to products. This seems a bit odd to me. In the two-set case, I would think terminology like "relation from $S$ to $T$" or "relation between $S$ and $T$" might be appropriate. As for the general case, where do people tend to discuss relations among elements of infinitely many sets as relations? Doesn't it make sense to just switch entirely to set-theoretic terminology at some point, unless perhaps you're doing some weird set-theoretic thing that will require a completely different definition of relation anyway?--Dfeuer (talk) 18:33, 10 January 2013 (UTC)


 * What's your point? --prime mover (talk) 19:09, 10 January 2013 (UTC)