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Since there are mathematics and mathematical-physics tags, I was convinced that it is perfectly legitimate to ask for a mathematically rigourous proof, i.e. a methematically correct, once given the assumptions made in physics on the mathematical objects used in the derivation, such as functions, domains etc. (reaching formulae supported and confirmed by experimental evidence by uncorrect mathematical procedures does not obviously fit the usual definition of rigourous that anybody having studied any mathematics is accustomed to).
Given the existence of those two tags, I was also convinced that it is perfectly licit to ask for a mathematical explanation of steps found in mathematical derivations of a certain physical law from given premises.
Such questions could well be also considered on topic on Mathematics Stackexchange, but I am not sure how much the users of that site may be acquainted with all of the physicist's assumptions made on a certain mathematical object, for ex. any assumption on a current density J (apart from the differentiability on a certain subset -not necessarily all of the domain where the function is defined-, which I think to be almost universal for any function in physics). On the contrary, I think that the users of this site, if they have already studied the derivation of a certain physical law, are likely to have asked themselves, or to the their Professors, or searched in booktexts for what justifies the equalities they have found when studying the derivation of that law.
Are such questions on topic here? If they are not, what other kind of question should the mathematics tag be used for?
This question, linked by Qmechanic, whom I thank, is quite related to what I am asking here, but I am focussing on a rather different issue: the "on-topicness" of questions about mathematical proofs involving mathematical objects subjected to assumptions defined by the methods of physics.
I would still be inclined to believe so, unless it had happened to me to become the target of insistent comments, accompanied by down/close votes, telling me that it is not clear what I am asking when, for ex., I ask for a proof whose steps can be mathematically justified (and where the answerer is hopefully willing to explain -as any standard mathematics or mathematical physics book does- to the asker and readers what mathematical results (= theorems, known properties,...) allows such steps when they are not trivial), or when I ask what mathematical result allows to commute an integral and a derivative sign.
I thank any answerer!