Span of Multisets
We haven't written anything yet, but me and a friend are looking at something we've called the 'span' of a set.
Basically, given a multiset, and a set of operations, what are all the numbers you can construct, when only using one number from the set exactly once. It seems to be quite a complicated question, and I think it would be part of Combinatorics/Graph Theory (since that's how we're rewriting a lot of the questions). We're going to work on it when we have some spare time and propably put the pdf on here when done.