 He worked on the theory of incidence algebras (which generalize the 19th-century theory of Möbius inversion) and popularized their study among combinatorialists, set the umbral calculus on a rigorous foundation, unified the theory of Sheffer sequences and polynomial sequences of binomial type, and worked on fundamental problems in probability theory.His philosophical work was largely in the phenomenology of Edmund Husserl.Rota died of atherosclerotic cardiac disease on April 18, 1999, apparently in his sleep at his home in Cambridge, Massachusetts.