[jimwhitson]

Abstract/linear algebra. I enjoyed it thoroughly at university but it never seemed very useful in practice, until I had to deal with a complex problem of manipulating graphs. All of a sudden I found semi-ring homomorphisms coming in handy again, and on the back of that realisation have made some very cool software.

The risky learning curve came when I had to revise all the stuff I'd half-forgotten, taking a few weeks out to do nothing but maths, in the hope that my hunch would pan out. It did pan out, and now I'm settling down to the long hard slog of exploiting it properly.