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The purpose of this question is to link to various old (say $\succeq 1$ year old) closed posts, mostly by no longer active users, most of which are reasonable candidates for deletion, and a few for reopening, while it (in most cases) makes little sense to keep them closed and undeleted.

If you have enough reputation, you should be able to cast a deletion or reopening vote ($\ge 10000$ for deletion [needs 3 votes], $\ge 3000$ for reopening [needs 5 votes]). The deleting option is present under some conditions: for closed questions I think it's automatic if the score is $\le -3$, and in other cases depends on parameters (OP's rep? age of the question?) in some way I don't know (edit: some info here); also at $\ge 125$ reputation you can downvote, which in some cases can eventually lead to automatic deletion.

(I'd avoid here very active questions for which closure may have been disputed.)

I'll use answers to make such partial lists (usually of posts in a given subject, e.g., detected by some given tag). (Of course one can post "real" answers!) At some point I'll make updates indicating deleted/reopened questions, if applicable.

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    $\begingroup$ I'll just mention that there were some discussion related to manual deletions on MathOverflow: Should we try to re-start manual deletions or is the situation fine anyway? and Decluttering MathOverflow. +1 from me - since this seems like a reasonable thing to discuss. $\endgroup$ – Martin Sleziak Feb 8 at 14:20
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    $\begingroup$ Sometimes keeping closed questions undeleted may discourage similar questions in the future. An argument for neither reopen nor delete. $\endgroup$ – Gerald Edgar Feb 8 at 14:41
  • $\begingroup$ I don't see what's wrong with "neither". There are several bad questions that have received good answers worth of archival. $\endgroup$ – darij grinberg Feb 28 at 19:04
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Here are the closed questions tagged (at this date) with the deprecated .

finite index, self-normalizing subgroup of $F_2$

Why do we not lose any generality by proving it only for finitely generated groups

(not closed but downvoted) Group associated to the monoid $({\cal P}(X\times X), \circ)$

Making idempotent element by a relation

How to solve this system of equations?

How to show the set $\operatorname{Hom}_K(L,\bar{K})$ of all $K$-embeddings of $L$ is partitioned into $m$ equivalence classes of $d$ elements each?

Is the "algebraic closure" of the quaternions, finite dimensional?

Is there any construction of infinite dimensional algebraic division ring?

Polynomial constraints triggered by irreducibility

Does this solution guarantee $det(A)=0$ where $A\in M(R)$?

Is $K\cap \langle H\cup N\rangle‎\subseteq‎ \langle H\cup (K\cap N)\rangle$?

Properties of colon ideal

Diagonalize the simultaneous matrices and its background

Learning Algebra & Group Theory on my own

now deleted https://mathoverflow.net/questions/47534/unit-ideal-in-non-commutative-rings

now deleted https://mathoverflow.net/questions/157122/algebra-generated-by-a-tree-edit

now deleted https://mathoverflow.net/questions/127611/an-exercise-about-tor

now deleted https://mathoverflow.net/questions/266080/does-alexander-whitney-formula-imply-pythagoras-theorem

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    $\begingroup$ mathoverflow.net/questions/88660/ring-of-a-spectral-space is currently open: was it closed when you wrote your original list? $\endgroup$ – Yemon Choi Feb 9 at 20:02
  • $\begingroup$ @YemonChoi no, it was never closed. -- You can see this in the timeline, which is now linked to below the vote score. $\endgroup$ – Stefan Kohl Feb 9 at 20:11
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    $\begingroup$ @YemonChoi thank you! it seems to be a copying error (it's neither downvoted nor closed, nor any particular reason to include it here) – this list was manually picked among the list of 260 question tagged 'abstract-algebra'. The given item is now deleted from the above list – despite being copied here since yesterday it duly got no closing vote. $\endgroup$ – YCor Feb 9 at 20:25
  • $\begingroup$ There is a good answer ato mathoverflow.net/questions/144341/… . Why are we voting for deletion?? $\endgroup$ – darij grinberg Feb 28 at 19:02
  • $\begingroup$ @darijgrinberg I expanded Lev Borisov's comment, which gives the immediate solution to the exercise (it works without assuming that $A$ has the given special form). The answer seems to be a more computational way to obtain the result without using the adjugate matrix. Whether this should be closed/deleted is maybe questionable, but it's the whole point of voting. $\endgroup$ – YCor Feb 28 at 19:40
  • $\begingroup$ @YCor: oops! You're right, the problem is trivial. I mistook it for a different problem, where the kind of induction in the answer is the best argument. $\endgroup$ – darij grinberg Feb 28 at 21:27
  • $\begingroup$ I don't see a good point for deleting this, though. Questions about understanding issues in research papers have been considered legitimate last time I checked, but even if they no longer are, the knowledge that has been generated shouldn't be destroyed. $\endgroup$ – darij grinberg Feb 28 at 21:29
  • $\begingroup$ @Gro-Tsen: To avoid loss of content, you may want to repost your answer from mathoverflow.net/questions/127611/an-exercise-about-tor at math.stackexchange.com/questions/1437800 (which asks the first part of the question; the second can be a postscript). $\endgroup$ – darij grinberg Feb 28 at 21:34

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