# Tag management 2017 [duplicate]

New year, new tag management thread.

Rules of the game are basically the same:

• A particularly bad tag (a rule of thumb: «if I can't imagine a person classifying a tag as either interesting or ignored, I'm getting rid of it»),
• A tag that should be a synonym of an existing one,
• A tag that used for two or more completely unrelated things,
• A need to create a new tag;
• Wait a couple of days before implementing a suggestion.
• After the problem described in an answer is resolved, please edit it to say so.

Also, note that one may use [tag:calculus] for , i.e. tags on the main site, and [meta-tag:discussion] for , i.e. for tags on the meta site.

• I recently proposed changes to tags concerning quaternion algebras here. I would appreciate more feedback in the form of votes on the three answers. – Viktor Vaughn Jan 17 '17 at 4:51

In my interest in mapping between a rectangle and its projection resulting in a quadrilateral I have learned of the important concept "homography". There are many homography questions here on the site but there is currently no "homography" tag.

Would somebody like to create this tag? If not, which tag should I use instead?

If you decide to create it, just search for one or more questions about homography and add the tag to that, those questions.

A very random sampling of homography questions, not necessarily the best in any sense:

I included a few that don't use the word "homography" anywhere on the page, perhaps because it's not a term everybody knows even when they come across such a problem for the first time, or possibly due to my own lack of deep understanding.

• Tags cannot exist without being associated with at least one question. If you can point out some questions in your post above for which you feel a homography tag might be suitable, it would make this process easier. – user642796 Aug 19 '17 at 5:27
• My god there's at least dozens. I'm on a temporary connection so I'll just add some random ones rather than necessarily the best ones ... – hippietrail Aug 19 '17 at 6:26

Here is a twofer: please merge (and optionally synonymize) into and maybe rename that tag to if we still can't have umlauts in tags.

Pluralize .

In connection with the discussion of projection tag I have noticed that there is the tag called .

This tag was very likely created with cartography in mind. The the first occurrence found by this SEDE query is the question How do great circles project on the mercator projection?. To clarify the distinction I will quote from the Wikipedia article on map projection: "More generally, the surfaces of planetary bodies can be mapped even if they are too irregular to be modeled well with a sphere or ellipsoid; see below. Even more generally, projections are the subject of several pure mathematical fields, including differential geometry and projective geometry. However, "map projection" refers specifically to a cartographic projection."

However, if you look at how the tag is actually used, there are currently 22 questions, most of them seem to be about projection in the sense of linear algebra. (There are only two questions which are also tagged cartography.)

What should be done with the tag ?

• Clarify in the tag-info that it is intended for cartographic projection and retag the existing questions in accordance with this?
• Remove the tag completely since it is likely to be ambiguous?
• Rename the tag in the way that it becomes less ambiguous - for example, or ?
• Embrace the chaos and use it for both meanings - projection in cartography and projection map?

If we keep the tag, it would probably be useful to create tag-excerpt and tag-wiki clarifying how the tag is supposed to be used.

The tag was created recently, here you can see a short discussion between me and the tag-creator. From this discussion it seems that the tag is intended both for functions and multifunctions. And also that the tag-creator would prefer to have separate tags for lower and upper semicontinuous tags.

My personal opinion is:

• I think that it might be useful to have a tag for semicontinuous functions. (It seems to be a rather common topic in real analysis and general topology. This class of functions is useful in some contexts.)
• I think that it would be better to have single tag rather than two separate tags. (The two classes of functions are rather close to each other. Moreover, having too specific tags is not ideal since for a question we can have at most 5 tags.)

However, before taking any further action, it would be nice to know what the community thinks about the tag(s) for semicontinuous function.

I will also point out that in the past some questions about this topic were tagged with (continuity) tag or no tag specific to semicontinuity was used. So maybe a few older questions will need retagging. (Probably it would be good to add tag mainly to the most relevant past questions about semicontinuous functions.)

EDIT: A clarification about the content of the tag was requested in the comments. My opinion is that the tag could serve for questions about semicontinuity of functions and multifunctions. (Notice that some people use the name hemicontinuity for the latter, and we have some questions using this name. But AFAICT the term semicontinuity is also used for multifuctions. For example Engelking's book, which I consider a classical reference, uses the name semicontinuity for set-valued mappings; see Problem 1.7.17.)

If needed, we can create a separate question for this. (We should do so if the discussion here becomes too long to be reasonably contained in the comments.) But for the time being, maybe it is sufficient if this is discussed in this thread.

• If anything, certainly having both upper and lower tags would be redundant. But now, is this going to help searching? Probably not as much, since typing "semicontinuity" or "semicontinuous" is likely to give a full spectrum of the results. Is this going to help calibrate the level or clarify the topic? Also probably not, since the word "semicontinuous" is likely to appear in the title anyway. So, what are the pro-arguments, again? – Asaf Karagila Nov 21 '17 at 12:35
• @AsafKaragila If we are discussing usefulness for searching, you should probably add "semi-continuous" (and "semi-continuity") and also "hemicontinuous" (and "hemicontinuity") to the list. (IIRC the latter is commonly used for semicontinuity of multifunctions.) – Martin Sleziak Nov 21 '17 at 12:38
• Okay, you didn't mention that hemicontinuity should also be covered by the tag. Which makes it questionable if semicontinuity is the right name. – Asaf Karagila Nov 21 '17 at 14:06
• @AsafKaragila Maybe it's because I talk with people in different circles,but I know the notions which are called upper and lower hemicontinuity in the Wikipedia article under the names upper and lower semicontinuity. So if we are talking about multifunctions, the terms hemicontinuity and semicontinuity seem to be both used quite commonly. (At least based on what I have seen.) – Martin Sleziak Nov 21 '17 at 14:41
• Well. I only talk to people in triangles and dodecahedrons. So we certainly don't talk to the same people. The question I have, if so, is what exactly this tag will include, then? – Asaf Karagila Nov 21 '17 at 14:43
• I'm afraid this answer does not get the exposure it deserves here. – Asaf Karagila Nov 22 '17 at 10:13
• @AsafKaragila To be honest, that is probably true for any of the answers in this thread. But if you think that it is a good idea, feel free to go ahead and start a separate question about this. (Or simply try to badger me a bit until I do so, if that's the preferred course of action.) – Martin Sleziak Nov 22 '17 at 13:14
• Probably the latter, or I just cave and agree that this is a good tag. But let's give it a few more days for everyone involved (i.e. you and me) to see what's going on. – Asaf Karagila Nov 22 '17 at 13:16