2:12 PM
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Today I posted the question below: Challenge: find the order of an element in a Finite Abelian Group with a given presentation I was surprised to find that there is no such thing as a challenge tag and took the liberty to tag the question as such anyway. It is meant to indicate that the answer ...

5 hours later…
7:05 PM
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Let $a, b, c, d, e, f, g$ denote integers, with $a$ and $c$ both non-zero, and consider the matrix $$M=\left(\begin{matrix} a&b&d&f \\ 0&c&e&g \end{matrix}\right)$$ Let $G$ be the quotient of the group $\Bbb Z^2$ by the column space of $M$. That is, $G$ is presented by two generators, say ...

A you can see in the meta question above, the tag was created by Arnaud Mortier.
I see a brief discussion on this tag in this room: chat.stackexchange.com/transcript/3740/2018/12/16
Dec 16 '18 at 16:56, by Martin Sleziak
In fact, it seems that also tag was created in the past (and then removed): https://math.stackexchange.com/posts/1365280/revisions https://chat.stackexchange.com/transcript/3740/2015/7/18 https://data.stackexchange.com/math/query/927958/posts-which-had-given-tag-in-th‌​e-past-including-current-tags?tagname=challenge
The tag with this name is also mentioned in Mark Bennet's answer: Using Math.SE for contests
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In a sense every question on the site is a contest - at least that is how the rewards work, to try to motivate users to produce the best answer. There is general encouragement to work together with others (through comments and edits) and subsequent answers which are essentially the same as previo...

Here are a few SEDE queries, but it seems that there was only a single occurrence in the past: data.stackexchange.com/math/query/927958/… data.stackexchange.com/math/query/883845/… data.stackexchange.com/math/revision/1038474/1282557/…