How to tell someone, that someone else "can"not answer a question I asked multiple times? I work at a unversity in the physics department. Since quite some time, I have a very specific question regarding some equations and how they were rearranged into some result. I have asked my Professor regarding this several times, but I have never been able to get a clear answer and at this point I am &quot;too afraid to ask once more&quot;. Of course, I am not really afraid but I have asked this so often that at some point it becomes somehow weird, especially since this is only some detail.
Now I have been assigned with supervising an undergraduate, who will write his Master's thesis about a topic which is very similar to the one I am working on. I can already see that he will come across the same question in the near future and there are several possible things I could do to answer the question if it arises.

I could simply tell him that I don't know the answer. This would of course be OK but does not actually solve the problem. Especially since for his work a better answer to the question I am trying to get an answer to might be required.
I could tell him that I asked this several times to the professor, but he either did not understand or was unable to provide an answer. This is the more factually correct answer but it sounds a bit accusatory and could put me in the position that the student tells the professor about this since I would be the first one whom he would ask and who should be able to provide an answer.
I could ask the co-authors. In this case, the question would be whether I inform my professor about this. If I do so, he will ask me why and I probably end up in a position where I will have to ask the same question all over again, not to mention that he did not answer the question in a way I understand.
This is somewhat of a frame challenge, but I suggest you try asking the question on Physics.SE (or Math.SE, or even MathOverflow, depending on where it fits best) so that you can become the person who can answer the question.