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 probably put the pdf on here when done.

If you want to discuss this topic, feel free to leave me a message or discuss on twitter.