I have read a recent paper (actually not published yet but is accepted for publication). In this paper the authors consider two scenarios and discuss performance of the considered scenarios. I am 100% sure that one of the parameters of the performance remains same for both the scenarios but the author provided two lemmas that contain two different mathematical expressions for that parameter. They have stated in the paper that those expressions are one of their main contributions. I can prove that for both the scenarios the performance parameter remains same and my expression is less difficult to compute and more nice looking. So, my question is that should I consider writing a comment paper?
If your claim is valid, strong and holds for all possible kind of problems that could be supported by the lemmas given by the authors, then you should do the following:
As the paper has been accepted (you should wait. Because, the paper you read is the paper you read might be from author's personal sites or arxiv (as I assume)). So, once the journal/conference publishes the accepted paper online, you could write a comment paper/a short paper based on your claims.
Without online publication by the journal, it is really difficult for you to talk to anyone. I guess.