Voting to close a question as off-topic because it's a notorious open problem

Question

I just voted to close a question which was equivalent to Goldbach's conjecture. When I had to give the reason, I didn't really know what to pick.
Off-topic? It is definitely a research question, so...
Duplicate? Well, hopefully not, at least not exactly.
When it was closed some hours later, I found out that three of the other voters had checked "off-topic - See Math.StackExchange to ask general questions in mathematics." Which is IMHO again not the best way to deal with it.

May I suggest to add another pre-defined possible reason for "off-topic"? Something like
"This question is identical or very close to a notorious open problem."

Answer 1

A research-level question can also be off-topic if the community has chosen to restrict the scope of the site so. Our help pages state that questions about open problems are off-topic, so closing because of that is fine. The standard off-topic closure reason is misleading, so I suggest you vote to close under a custom reason. Choose the option "Other" under "Off-Topic" in the close vote dialogue and write what the problem is. I think this particular reason to close is not so common that we would need a separate closing reason.

I think the particular question would be much better at MSE, which is why I voted to close for that reason. It seemed to me that it would be an acceptable answer that the OP's conjecture turns out to be equivalent with something famous and open, but such an explanation should rather be given at MSE than here. For MO audience I feel that the problem is trivially equivalent to Goldbach's conjecture, but the OP and the average MSE user may not consider it as trivial.