Some believe Elliott Carter to be the greatest American-born composer; no less an authority than Igor Stravinsky considered Carter's Double Concerto (1961) the first true American masterpiece. Certainly he was one of the most intellectually rigorous composers of his or any other time. Carter's work is characterized largely by its rhythmic complexity, befitting a man who was a serious student of mathematics. Initially a neo-classicist, Carter went on to embrace dodecaphonics and serialism. Carter invented a concept he called "metric modulation," which musicologist/composer Nicolas Slonimsky described as a system "in which secondary rhythms in a polyrhythmic section assume dominance expressed in constantly changing meters, often in such unusual time signatures as 10/16, 21/8, etc." Adding to his importance as a composer was his long life and prolific output; Carter remained vital as a composer even after passing 100 years of age, with little or no discernible decline in the quality of his work.