[wakawaka28]

I find it annoyingly unrealistic to demand that nobody publish solutions, especially interesting ones like this. Some people will always do it anyway, and once they do, any further resistance to such publications is pointless. If you want something secret, your ultimate mistake is publishing the problems in the first place. For a problem set as old as Project Euler, it's not reasonable to expect solutions to not be out there, and it is ultimately on the honor system whether a problem solver cheats or not.

[xzyyyz]

It is called the honor system. But you're right: some people... And no, Project Euler does not demand. You can be sure, nobody is going to be sued. It asks politely to show respect to those who created the project, and to those who did not solve the problem.

Besides other things, there is a place to publish interesting solution: it is Project Euler forum to discuss solved problems.

[wholinator2]

Demand is a strong word. I do think it's entirely reasonable to ask, especially around the time the problems were first published. Things have changed on the internet