I do not understand why this question were reopened? Could anyone explain me it please. I voted to close it and it seems to me that it is "off-topic". If this is not so, then I don't see any point in asking people to add their work.
Would it kill the OP of that interesting question to even give the SLIGHTEST evidence of an attempt to solve the question? It's a pretty low bar to set. If they won't do that, then I see zero reason to keep the question open.
In contrast, I see plenty of reasons to keep the question closed, despite it being interesting from a purely mathematical perspective: closing the question sends a message that users should put in at least a minimal amount of effort when asking questions, so as to prevent this site from becoming a homework answering service. It also says that we don't discriminate: just because you personally find it interesting doesn't mean someone else does. If you encountered a question that was posed in such a manner that merely asked "prove the Pythagorean theorem," would you not close it for its lack of original effort? Triviality or non-triviality is subjective and as such should not be used as a basis for selecting one question to keep open and another to keep closed. However, asking the submitter to just show something that they tried honestly is NOT such an awful burden. Some of you are acting like we're asking them to solve the question themselves and publish it in a peer-reviewed journal before posting it to math.SE.
Bit long for a comment. A large segment of similar posts are contest questions rather than homework, maybe recent, maybe older. Sometimes these catch my eye owing to subject matter, but mostly not. More to the point, the same OP has often gone and posted three more while others look at the first. We have one guy with 434 (mostly) contest questions, another with 598 (but about 200 answers). I've just stopped opening these. I don't want to answer someone who is simply not paying attention, or maybe does not care about the material, just wants to post something. Meanwhile, as far as my asking about the work put in by the OP and the background/ motivation of the question (my usual concerns), there is not going to be anything satisfactory; the student has generally not been taking a class that discusses precisely related material.
It occurs to me that if I am not opening a question, I am also not voting to close it. Well, easier on my nerves this way.
Edit, Tuesday, August 19: it is a red flag with me if someone just dumps a question and does not respond to comments. I indicated this once on MO, one of the moderators suggested I was too quick to take offense, 24 hours seemed a more reasonable time window for responses by the OP. Well, 24 hours have passed for the question on Main under discussion.