# Should we delete or reopen these old posts?

The purpose of this question is to link to various old (say $$\succeq 1$$ year old) closed posts, mostly by no longer active users, most of which are reasonable candidates for deletion, and a few for reopening, while it (in most cases) makes little sense to keep them closed and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

• 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. – Martin Sleziak Feb 8 at 14:20
• Sometimes keeping closed questions undeleted may discourage similar questions in the future. An argument for neither reopen nor delete. – Gerald Edgar Feb 8 at 14:41
• I don't see what's wrong with "neither". There are several bad questions that have received good answers worth of archival. – darij grinberg Feb 28 at 19:04

• @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. – YCor Feb 28 at 19:40