But it should be put on hold for duplicate. Why on hold for off-topic? It is clearly about math.
I initiated the close vote, and selected the reason "not about mathematics." The original asker had blanked the post, editing it with a message about a desired that the question be deleted; since it wasn't a question anymore, I voted to close it as not on-topic.
Another user has since reverted the edits and restored the content in the question, and so I agree that closure-as-duplicate would be a better option. As far as I know, the only way to change the closure reason would be to reopen the question and then close it again. Since I doubt that it would be possible to find 5 users to vote to reopen, I'd suggest flagging the question for a moderator and linking to a proposed duplicate; they can open and close questions unilaterally.