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@SylvainJULIEN: Apologies for the delayed revert. But my recent preliminary computations suggest that we can take $K=k+1$. I am still working out the details. Will let you know.
I do not have much background in higher calculus too, but I cannot help but say: A non-commutative generalization of the arithmetic derivative would be very nice! =) Do let me know what you find out.
@MarkVSapir: If you do not have a mathematical proof, then I find it surprising that you (as a professional mathematician) can plainly state that there are no odd perfect numbers.
Ohh, yes! But then that does not require proving that $m^2 - p^k \neq \square$! Ohh my goodness! If you could just write that out as an answer, I will be more than happy to accept it (and award the bounty to you), @AlexRavsky =)
Thank you for your comment, @AlexRavsky! I can see that your claimed inequality $p^k < 2(m - 1)m$ is equivalent to $m^2 - p^k > 0 > 2m - m^2$, which is always true. (We do know, however, that $p^k < m^2$ always holds. So does that take care of your concern about the failure of my "finishing arguments"?)
@AndyPutman: In that case, please do allow me to edit my question, to conform to MO policy, so that it does not look like a "request to check completeness of proofs".
@JoshuaZ: I appreciate your comment, Josh! I was hoping someone with more experience on the Euler totient function would be able to settle this one. (This question was mainly triggered by an inquiry that I received via e-mail.)