Greeks continue to contribute to science and technology in the modern world. John Argyris, a mathematician and engineer, was among the creators of the finite element method and the direct stiffness method. Constantin Carathéodory made significant contributions to the theory of functions of a real variable, calculus of variations and measure theory, credited with the introduction of several mathematical theorems. In physics, John Iliopoulos is known for the prediction of the charm quark and the proposition of the GIM mechanism, as well as the Fayet–Iliopoulos D-term formula, while Dimitri Nanopoulos is one of the principal developers of the Flipped SU(5) model.