The theory of transformation functions, as has been developed by Schwinger, Feynman, Dyson and others, is investigated systematically with mechanical models, keeping close analogy to classical theory; and a new light is thereby thrown on the relation between the Lagrangian and Hamiltonian formalism. In these investigations the idea of chronological ordering introduced by Dyson on the one hand, and a differential operation with respect to coupling constant (or some other parameters) on the other, play a decisive part. The formulation is extended to a point where dynamical systems with higher time derivatives as well as “non-local” systems (with integral equations of motion) are included.
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Y. Nambu (1952) studied this question.