In these two years I'm always trying to understand how to write well my questions.
Some of my Physics questions necessarily also require mathematical knowledge that I don't have at the moment and the user of Physics.SE. invite me to write a question on Mathematics.SE.
But some users of Mathematics.SE ask me, instead, to write questions about physics, although they contain mathematical aspects, that they are not welcome on the site Physics.SE. Why?
This is a "border case" problem, i.e. if your question is partially math, partially physics. Math is possible without Physics, but practically all Physics use Math.
Unfortunately, the topicality of the sites is decided by their communities, independently from other sites. Thus, there might be questions which are ontopic on both sites, and also questions which are offtopic on both (while they should be ontopic on at least one of them).
Recently the MathSE got a far more closed direction. The PSE was always restrictive, but at least consequent (with an exception). The good thing is that there are not too many such "blind spots", i.e. topics which are offtopic on both sites. The bad thing is that the visible cross-site cooperation to solve these "blind spots" is little or non-existent.
The important thing is: if you ask a question on the MathSE, it is a Math question from that moment. So less non-mathematical content has it, so better is it. Similarly, if you ask a PSE question, commit everything to not look it a MathSE question in physics disguise.
It is also very important, your questions can be closed on many reasons. Carefully watch all the feedbacks you get, and follow them even if they look hairsplitting. Because it happens often, that the comments are wanting meaningless "improvements". Don't argue them, follow them. Note: they could vote you down/close even without a comment. So it is better to follow them. Furthermore, help them to understand, that you know, what are you asking about, and that you would understand also the answers (links, references, own research). Make the mis-interpretation of your question so hard as it is possible. If you get the comments and answers, react everything and be cooperative with everybody, even if they are clearly visibly.... well, not very useful comments.
If a question is a border-case off-topic, its chance is much better if it is HQ in all other sense.
Some suggestions:
Stop using your "student" notes as a reference. We do not care where your question comes from, and unintelligible scribbles from a student are of no use to us.
Stop using books/webpages in Italian as a reference. This webpage operates in English, and if that does not work for you, you'll have to find some Italian physics community somewhere online. A screenshot of your book, with a rectangle around an equation, is even more useless the equation itself (cf. pictures vs mathjax).
Stop playing the victim; stop using the pathetic argument (in the formal sense of the word). We judge posts, not users. If your posts are bad, they'll get downvoted/VTCd. Learn from your mistakes and move on.
Your question is currently extremely unclear (potentially because you're over-relying on unreliable machine translation?), but the three initial versions of the text contain some kind of version of the following,
In my view there is no homogeneity of views between one site and another.
Why is there no coherence between the two web-sites?
as well as the (currently utterly unclear)
But some users of Mathematics.SE ask me, instead, to write questions about physics, although they contain mathematical aspects, that they are not welcome on the site Physics.SE. Why?
and, while this might not reflect what you actually wanted to ask, there's enough here to address a key point:
They are run by separate communities, serving different audiences for different topics and in different environments, with vastly different histories. We're bound by the (very loose) framework of the Stack Exchange ethos and overall rules, but anything above and beyond that is a bonus. In particular, this means that the fact that a certain type of question is on-topic at the Maths site does not guarantee that it will be on-topic here, or vice versa.
There also isn't a clear delimiting line that says "questions on this side go to Physics and questions on this side go to Maths", in any shape or form. Some questions might be on-topic on both sides, while some questions might not be on-topic on either site. In other words: "this question is too physicsy for Maths SE" does not guarantee that it will be on-topic on Physics, or vice versa. If that's what's bothering you, then I'm sorry, but this site does not aim to be all things to all people, and it's entirely possible that we're not what you're looking for, and that you'll need to find that elsewhere.
Now, it can happen that someone on Physics will tell you something like
this question is too mathsy for Physics SE, you should try asking it on the Maths site
but that your question then gets a bad reception on Maths. (Let's leave aside, for now, the (entirely real) possibility that the bad reception is due to, say, a bad use of language on your part.) If this is the case, you should not interpret those comments on PSE as being "guarantees" that MSE will work out: instead, they're clear indicators that the question is too mathsy for our scope, but (unless the user in question is demonstrably a frequent user of MSE) only tentative pointers of other places to try. And again: the fact that it's too mathsy for PSE does not guarantee, in any way, that it will be on-topic at Maths.
And finally: Language Matters. If your questions use unclear language, then they are more likely to be closed as Unclear What You're Asking, and they are more likely to be downvoted.
Those are core, integral parts of how the SE Q&A engine works, and they're one of the key reasons why SE shines in providing good-quality content. If these aspects bother you, then you should look for other platforms that don't have content quality as a key tenet.
The same holds for text that is not in English: the language of this site is English, and including crucial parts of your question in other languages is going to lead to a bad reception, from downvotes to closevotes to comments telling you not to do that. If you want to avoid that bad reception, then post only in English, and only include other languages when absolutely crucial and accompanied with a suitable, high-quality translation. To be clear: machine translation is generally not of sufficient quality to be posted directly, particularly on technical subjects like physics. You are responsible for the clarity of the language that you post here, and if it's unclear enough to merit downvotes or closevotes, then edit it to clarify it, or accept the response. Downvotes are not a personal indictment against you, they are indicators about the content of the post.
Similarly, on this site there is a very strong consensus that screenshots of text are off-limits here, which means that posts with such screenshots are similarly subject to a bad reception. If you don't want that bad reception, then follow the site consensus.
And finally: same as with Maths SE, there is no requirement of a cohesive policy with TeX SE. Some things might fly on TeX SE that are not OK here -- in which case, good for TeX SE! but that still doesn't mean that they can fly here. Your text might be clearer on TeX SE (where posts are generally aided by the fact that they contain code, which is much more unambiguous that technical text on physics), but that doesn't mean that text of similar quality passes the standard clarity bar here.
