# Correcting a conference paper error in the journal version

In a conference paper, in order to prove the $\mathsf{NP}$-completeness of a problem, I wrote the stupid sentence "It is clear that the problem is in $\mathsf{NP}$. So we will prove that it is $\mathsf{NP}$-hard". In fact, it was not clear at all. It even seems to be an open problem. For the targeted audience, it's not a big deal because the main result is the $\mathsf{NP}$-hardness, and thus the impossibility "difficulty" to design a polynomial algorithm.

But, of course, I want to correct the mistake in the journal version.

My question is: In a journal paper, how to point a mistake in the conference version and correct it?

Should I write: "There is a mistake in the conference version: ..."? Or should I state the correct result without saying that the conference version was wrong? Any advise is welcome.

• In addition to correcting the mistake in the journal version, I think it is helpful to post the postprint version of the paper in a repository (e.g., arXiv) with the corrected statement and indication that the publisher version contains an error. (If the paper has already been deposited, you can update the deposit to do this.)
– a3nm
Apr 24 '17 at 17:08