First, thanks to GertVdE for going through the tags and proposing changes. AronAhmadia and I have looked at GertVdE's list, and now I'm going through and cleaning up and merging some of the tags. The clean up will be done in stages; this first stage has been proposed on 17 March, and will stay up for a week for comments.


  • because its suggested usages make it seem like a "meta tag"; such tags are discouraged (see comment thread associated with Dan's answer)


Mostly for disambiguation.

Merges (Renames if the target tag does not exist)

Generally speaking, tags with fewer than 5 mentions are being merged into larger, relevant categories (there may be a few exceptions). Singleton tags are discouraged, but kept if they don't fit nicely into larger categories.

  • $\begingroup$ Are you merging or just aliasing these tags? $\endgroup$ Mar 20 '12 at 17:10
  • $\begingroup$ It would be a merge, using the tag on the right as a synonym for the tag on the left. In every case, the idea is to take a subcategory tag and retag it as a member of a larger category. $\endgroup$ Mar 20 '12 at 17:21

I think the proposed -> merge is probably not a good idea. Things besides matrices can be symmetric. For example, questions about handling exchange symmetry in quantum mechanics or using symmetries to reduce problem size might use that tag.

  • $\begingroup$ That's a good point. If we're going to think of symmetry that broadly, then it starts to look like a "meta tag" because symmetry is just modifying other tags, and there will be a grab bag of questions from quantum mechanics, fluid mechanics, and physics, in addition to linear algebra. As it stands, the proposed rename will be taken off the list; the one question it corresponds to is, in fact, a question about group theory. Mea culpa. $\endgroup$ Mar 20 '12 at 14:39
  • $\begingroup$ Symmetries can be discussed in general, i.e., symmetry groups. $\endgroup$ Mar 20 '12 at 17:05
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    $\begingroup$ I think the symmetry tag is just too broad to be included as a tag. $\endgroup$
    – Paul Mod
    Mar 20 '12 at 20:26
  • $\begingroup$ @Paul: I disagree. I think that tag would be a good fit for questions about handling symmetries of the things being simulated. $\endgroup$
    – Dan
    Mar 20 '12 at 22:23
  • $\begingroup$ @Dan: I think what Paul is getting at speaks to my previous comment. If you're talking about symmetries of something, then you can use the tag for that application, which seems to be why no one has used the symmetry tag for linear algebra. When discussing point symmetries, one could use the group theory tag. Regardless, symmetry just becomes a tag modifying other tags, and that sort of tag usage is discouraged. $\endgroup$ Mar 22 '12 at 17:17
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    $\begingroup$ @Geoff: Fair enough. Removing the 'symmetry' tag sounds like a good idea. $\endgroup$
    – Dan
    Mar 22 '12 at 19:24

Renaming heat-diffusion to diffusion is a poor choice. Heat-diffusion is fundamentally different from mass diffusion in molecular dynamics simulations, for instance. It is dangerous to merge these tags "permanently" as they may become quite disparate over time. E.g., B3LYP is a very popular density functional and may attract many questions over time. Do we have a categorization scheme, as in B3LYP questions are a subset of quantum DFT questions, which are a subset of quantum mechanics and general DFT questions?

  • $\begingroup$ I don't follow your reasoning. On a macroscopic level, the analogy among transport of energy (heat diffusion), mass (mass diffusion), and momentum (fluid mechanics) is routinely used in chemical engineering textbooks of transport phenomena. Bird, Stewart, and Lightfoot use this analogy verbatim; Deen also uses this analogy. The main difference is boundary conditions; the diffusive terms in each equation are otherwise the same. Are you talking about the microscopic versions of these phenomena? If we have enough questions about heat diffusion, it would merit its own tag. $\endgroup$ Mar 20 '12 at 17:19
  • $\begingroup$ @GeoffOxberry: I could have sworn I had already answered to your comment. Yes, it's the microscopic regime. The propagation of intensive properties is an ongoing subject of debate. I'm not aware of a proper (microscopic) definition of temperature for non-equilibrium states. $\endgroup$ Mar 28 '12 at 16:33

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