What to do about a citation that may be wrong I am writing a paper and I needed to cite some fact. I had difficulties finding something that I could cite. I have found a paper that also cites this fact and they cite some other paper without further explanation. I have gone through this paper that they are citing and I cannot find how the fact that I want could follow from it. It seems somewhat relevant but going through all the theorems, none of them seems like they could imply what they claim. I do not have a deep understanding of that paper and it seems it would be rather difficult for me to carefully read the whole thing.
I suspect that this result should be somewhere in the literature but cannot find it.
I have emailed the authors but did not get a response yet. I have contacted the corresponding author who forwarded my email to another co-author so I know they have received my email.
At the same time, it is not too difficult for me to prove the claim myself. Therefore, I need the citation only to give credit to the original author and to make my paper a little shorter (not very important as there is no strict page limit where I intend to submit).
What should I do if the authors do not respond before the conference deadline? I can think of the following:

Say something along the lines of &quot;paper A cites paper B but it is not clear how the fact follows from B. For completeness, we provide a proof here&quot;
Ignore that paper, as I was not able to determine whether the citation is correct. Do not claim my proof to be original but rather say I was not able to find any reference but I suspect that it has been proven before.
Just cite the paper they cite

Of these, (3) is clearly not good. Is it OK to choose (2)? Should I contact the authors once more before the conference deadline and inform them about what I intend to do?
A polite way to deal with this is to say &quot;We need the following known result (c.f. [A] and [B])  and include the proof for the convenience of the reader.&quot;  If someone writes you to clarify before this is finalized, you can revise.
If you are worried about giving credit that is inappropriate, take a moment and reflect on how citation counts are often misleading.  Your primary concern is the reader (including referees) and the above allows them to be as informed as they need to be.
If someone (say a referee) asks you to cite [B] for the proof, ask for a page or theorem reference.