I am a bit confused by the close reason for https://mathoverflow.net/questions/154235/minimum-l1-norm-may-not-obtain-the-sparsest-solution?noredirect=1#comment395410_154235.
It is an easy question that the reader could have solved by reading carefully the statements of the theorems, I agree, but it is nevertheless a question encountered while reading a research paper in mathematics. Hence I am unsure about the factual correctness of the statement "This question does not appear to be about research level mathematics within the scope defined in the help center."
"Help center" here links to a page stating:
What kind of questions can I ask here?
MathOverflow's primary goal is for users to ask and answer research level math questions, the sorts of questions you come across when you're writing or reading articles or graduate level books. Of course, individual questions don't have to be worthy of an article, and they don't have to be about new mathematics. A typical example is, "Can this hypothesis in that theorem be relaxed in this way?"
I think the main source of confusion here is that the closure reason "this question is not research-level maths" is used for two different reasons:
this question is off-topic because it's not about maths --- for instance, it's about physics, or about vacuum cleaners.
this question is a reasonable math question, but it's too easy for a professional mathematician.
Maybe we need a closure reason that is a nice way to tell "too easy, sorry". Something like "this question could be readily answered using standard material for an undergraduate student in mathematics, and so it is not of interest here". Or something better.
There is also an additional reason of confusion in this policy: what is an easy question for a graduate student in, say, algebraic topology, could be a challenging one for a professional researcher in, say, PDEs. A professional mathematician studying topic X could come up with a perfectly reasonable question about topic Y which is "standard" for someone using it, but yet not so easy that it is "undergraduate material". This is material that would be considered "too easy" at a conference on topic Y, but not if you ask another mathematician.
Are these kind of questions on-topic?
I see many questions of this kind on MO. I suspect that even more of them are so, for an expert in the suitable field, and it's just that I cannot recognize them as such. In my point of view, this is often the kind of questions that I want to use MO for. It saves me the hassle of locating in person an expert on topic Y (which my university could not have) and going to his room to ask.
On the other hand, I could see an argument also for banning this kind of "reference-request" questions. If it's something that you could have answered simply by locating a researcher in algebraic topology, any researcher, why are you bothering an entire online forum which includes several Fields medalists?
What would be left? Questions that are really "research-level", as in: they would not be trivial even for an audience comprised entirely of experts in the field. This would reduce a lot the traffic here, and also sectorialize it a lot. Everyone would understand only a very small subset of the questions. Many more questions would be closed. Actually, this criterion would sound very close to "if I you answer it, then close it instead of answering it".
There are two overlapping questions maybe, one about the closing reasons, and one on MO's policies on what is to close. I am sorry, but I find it difficult to split this up into two separate arguments.