It's not exactly common to find rock bands inspired by mathematical concepts, but Alt-J – a keyboard command that results in the delta sign – like the idea that the symbol is used in equations to show change. The reason to see them, though, has everything to do with their sound. They call it folk-step, which blends indie-rock, psychedelic folk and electronica into a mesmerizing hybrid on last year's eclectic debut, An Awesome Wave, winner of England's Mercury Prize.