There was a question (now deleted) which asked for books and resources on Number Theory and Geometry to help with programming competitions.
Can we make a place for things like this?
It isn't a 'Tips for golfing in language X?' question, but I think it (and similar) might work well in tandem with the others. Perhaps split up Number Theory and Geometry into separate questions?
There has been much discussion in chat and meta about preferring challenges that are not trivial, but remember, "The higher, the fewer."