Higher Derivatives and Canonical Formalisms

Path integral expressions for three canonical formalisms -Ostrogradski's one, constrained one and generalized one- of higher-derivative theories are given. For each formalism we consider both nonsingular and singular cases. It is shown that three formalisms share the same path integral expressions. In particular it is pointed out that the generalized canonical formalism is connected with the constrained one by a canonical transformation.