Jewish Problems - hard math problems with simple solutions

Discussed at great length on at least two previous occasions:

https://news.ycombinator.com/item?id=4759642 (399 days ago, 69 comments)

https://news.ycombinator.com/item?id=3096793 (794 days ago, 187 comments)

Comments there are, of course, closed, so if you have anything new or interesting to say then this is the place to do it. If you're interested in the thoughts of the HN community, there is already much to read.