8:46 AM
was created by Michael Hardy. Empty tag-info.
3

Charles Sanders Peirce wrote$^\dagger$ about an orthomorphic or conform projection formed by transforming the stereographic projection, with a pole at infinity, by means of an elliptic function. ("Conform projection" seems to mean what today we would call a conformal projection.) For th...

5 hours later…
1:54 PM
Somewhat related to tags - namely .
in Math Mods' Office, 3 mins ago, by Martin R
The [contest-math] tag was added to many questions recently, even old inactive ones, because a certain user "knows they look like a contest question" (http://math.stackexchange.com/questions/1274242/how-prove-fraca2ab2-fracb2bc2-f‌​racc2ca2-ge-frac/2073286#comment4258062_2073286). Does that make sense, even if the original poster does not mention a math contest or competition?
May I ask how you know that this (or the other > 10 questions where you added the [contest-math] tag recently) is a "Problem from or inspired by mathematics competitions" in the sense of math.stackexchange.com/tags/contest-math/info? — Martin R yesterday
@Martin R 2 Because I participated and I am teaching to contests and I just know how seems an olimpiad's problem. — Michael Rozenberg yesterday
This meta discussion had some comments related to contest math questions (and perhaps also to contest-math tag): Why a question without showing any work is getting upvoted?
Jyrki Lahtonen: I am somewhat in favor of various subcommunities, say, those forming around selected tags, within Math.SE developing their own norms. Enforcing such norms will mostly be up to the subcommunities themselves. It is good to have some common standards (enforced for example via our common review queues), but IMO the keen followers of a tag are best placed to judge many cases.
A good example of such a subcommunity is the one built around those tough definite integrals discussed recently.
zyx: It has never been the tradition in the contest problem community to provide more than the question, and a source for the problem/solution where known.
I do not recall a discussion on meta discussing specifically (contest-math) tag, perhaps with the exception of brief mention in this answer: Which questions falls under the tag 'problem-solving'?

2:38 PM
I would certainly agree that usage of tag is somewhat unclear. Perhaps with the exception of situations when the post explicitly says that the question was taken from a contest (or contest-related book or website).

3:08 PM
As I understand the tag info, the question must be related to an actual math contest problem (and for example, not a homework question). This math.stackexchange.com/q/855283/42969 for example could be a contest, a homework or an exercise from a book. Why add the contest tag if the author does not mention a contest? I would not bother for a single question, but the same user added the tag to many questions recently.

Clearly, the tag-info is rather old - it mentions now non-existent (homework) tag.
I am not sure to which extent this is an issue. But if we see that nobody will add some feedback on this here or in the mods' office, it might be reasonable to open a discussion about this on meta.
It seems that I am not that only one with that suggestion.
in Math Mods' Office, 2 mins ago, by J. M.
But you might want to cast a wider net of opinions by asking on meta.

1 hour later…
4:23 PM
has been created and relatively quickly added to 9 questions.
2

Let $G=(V,E)$ be a graph with no perfect matching. Then there exists a vertex $v$ such that every incident edge (i.e., every edge incident to $v$) is part of a maximum matching. I'm not sure how to prove this. How can every edge that coincides with $v$ be part of a maximum matching? It sou...

I guess that it is probably for the meaning from graph theory, rather than other meanings.
In economics, matching theory, also known as search and matching theory, is a mathematical framework attempting to describe the formation of mutually beneficial relationships over time. Matching theory has been especially influential in labor economics, where it has been used to describe the formation of new jobs, as well as to describe other human relationships like marriage. Matching theory evolved from an earlier framework called 'search theory'. Where search theory studies the microeconomic decision of an individual searcher, matching theory studies the macroeconomic outcome when one or more...
In the mathematical discipline of graph theory, a matching or independent edge set in a graph is a set of edges without common vertices. It may also be an entire graph consisting of edges without common vertices. Bipartite matching is a special case of a network flow problem. == Definition == Given a graph G = (V,E), a matching M in G is a set of pairwise non-adjacent edges; that is, no two edges share a common vertex. A vertex is matched (or saturated) if it is an endpoint of one of the edges in the matching. Otherwise the vertex is unmatched. A maximal matching is a matching M of a graph G with...
Matching is a statistical technique which is used to evaluate the effect of a treatment by comparing the treated and the non-treated units in an observational study or quasi-experiment (i.e. when the treatment is not randomly assigned). The goal of matching is, for every treated unit, to find one (or more) non-treated unit(s) with similar observable characteristics against whom the effect of the treatment can be assessed. By matching treated units to similar non-treated units, matching enables a comparison of outcomes among treated and non-treated units to estimate the effect of the treatme...
@darijgrinberg I see that you have created (matching-theory) tag. It might be useful to create also tag-wiki or at least tag-excerpt. It might help other users to use the tag correctly. Especially since this word is used in other meanings (economics, statistics - see here.) — Martin Sleziak 34 secs ago
Another tag shown among recently created tags is . It does not seem useful, so probably the easiest way to go is to remove it. (The question is also problematic since it is a "cross-site duplicate".)
0

Self-factorial number is the number whose sum of factorial of digits is equal to the number itself. But there are only a few amount of them. For example; $$1=1!$$ $$2=2!$$ $$145= 1!+4!+5!$$ So what is the fourth self-factorial number? How about fifth?