I just wanted to ask wether this problem has already been proved or not.

I know that there are 2 other posts that deal with exactly the same question, but I decided to ask it again, since they are too old (more than 5 years) and a new paper was published in 2015 on arxiv:

https://arxiv.org/abs/1601.02890

In spite that it's title contains the word revised, it does not seem to have been published. No "new mathematic" is presented, since it is a short paper. Any idea about it?

(Please, do not report this question as "duplicate", since we are working with new information that was not available years ago)