Is it allowed to publish contest problems on the internet without mentioning which contest it's from? Also, is it always allowed to publish a contest problem, as long as you mention which contest it's from?
Is it allowed to publish a non-contest problem without mentioning its source (say you've got it from a book)? Sometimes I see a problem occuring in multiple books, with neither one referring to the other (or any other). So, would it be allowed to put every problem from an entire book on the internet? Or half of it? I guess not. But on what grounds?