They begin to play: A perverse electronic drumbeat punctuated by abrupt bass notes, like a drunken man knocking over tables as he lurches out of a bar.  Broken chords speed up and slow down on the piano in an uneven cycle.  The drummer starts a more coherent beat, somehow meshing with the piano despite refusing to match its rhythm.  And the bassist unrelentingly hammers out that introductory pattern. Such is Vijay Iyer’s song “Accelerando,” which opened his trio’s show last Thursday night at the ’Sco. Iyer, who earned a degree in mathematics and physics from Yale before becoming a jazz pianist and composer, is fascinated by rhythm. Many of his compositions — and much of his improvisation — are based...