I am most fascinated by the topics which are at the borderline between pure and applied. Extremly pure math is a bit depressing to me: mostly extremely high amo

I am most fascinated by the topics which are at the borderline between pure and applied. Extremly pure math is a bit depressing to me: mostly extremely high amount of learning is needed to create anything interesting (math is extremely mature already) while there is no product to which you can point and say: it works! (Also, the working product can be appreciated by laymen, while a pure math paper can be appreciated by only those few who understand it.) At the other extreme of the spectrum: maintaining typical business applications in teams (originally programmed by other people) day by day is also demotivating. That's why my personal heroes are people like John von Neumann (and Alan Turing), who both worked on pure and very practical things.