I don't know the particular open/close history of the question, but I can't see any reason why it should be closed (and it is strange that it should have a negative vote total). It is a perfectly good question in number theory (in fact
a rather famous one!), and I would certainly vote to reopen it if it were closed.
Added: It seems that the question of MK that my answer responds to (and which was the one originally linked in this post) is in fact not the one that the post intended to refer to. This is just one reflection of the confusion that seems to be surrounding this issue.
Since I am adding to this answer, let me add a further remark: as far as I can tell, there are three MK questions involved in this discussion, of which two were substantial number theory questions and of which the third (which this post was intended to refer to) was about notation. The third was closed as non-constructive, quite possibly with solid justification. It seems to have been undeleted by error. On the other hand, I agree with Bill that the two number theory questions are reasonable, and should not have been deleted. I understand that they were deleted for reasons related to the automatic functioning of the software, but this seems to me to be akin to a case when an OP's question or answer becomes CW because of multiple edits. Certainly there have been cases when that CW status has been removed by a moderator because they deem it to be inappropriate, despite the software's automatic process; similarly, in this case it would seem to be silly to have solid number theory questions deleted just because of an automatic process.