Let us take as an example this question. Consider whether you think it should be considered off-topic (for being purely mathematical in nature).
Should one's opinion on whether a mathematical question be closed be based on what one finds interesting, the specific written rules of this site, a combination, or something else entirely?
I do not think this question is trivially answered by referring to the site tour (see KyleKanos's comment below) because the tour says that what's allowed is
Mathematics in the context of physics
Questions on mathematical details outside of physical context are off topic
The problem seems to be that there is enormous room for interpretation. A pure math question may have obvious physical implications even if the question doesn't explicitly reference those implications.
One approach is to take the question at face value and not allow for any benefit of the doubt with respect to implicit relevance to physics. With this approach, the example question linked above would be closed.
Another approach is to consider whether the mathematics in the question is directly relevant to physics or might simply be so common in physics settings that other users of the site would find it interesting and useful. From this perspective, the example question might be kept open.