The problems that I post here come from all sorts of different sources: from books, articles, websites, stories told to me by other people, etc. Usually I don't bother mentioning where the problem is taken from, as for the purposes of this website it is mostly irrelevant. However, if you believe that the problem, for instance, has an interesting historic background that is worth to be mentioned, let me know either through an email or in the comments to the problem and I will try to incorporate that information into the body of the problem.