Say I find a Math Proposition in another paper, which is just stated, and no proof (nor even sketch of proof) is given.
If I am writing a paper referencing that paper, is it good or bad (from reviewer's point of view) for me to fill in the proof of that proposition?
If I do so, what is a good way for me to indicate it in the paper? (that the proof written is by me, though the proposition is by the other paper)
The proof is not trivial, though it is not particularly difficult either.
Or should I just privately verify if the proposition is correct? And just cite the proposition (without proof) in my paper?
I am concerned about this issue since I followed the news story in the case of Grigori Perelman's proof of Poincare conjecture; the authors who filled in "gaps" in his proof did not go well with the reviewers and the general public. (https://www.newyorker.com/magazine/2006/08/28/manifold-destiny)