A General Variational Principle of Classical Field and Its Application to General relativity I

A general variational principle of classical fields with a Lagrangian containing the field quantity and its derivatives of up to the N-th order is presented. Noether's theorem is derived. The generalized Hamilton-Jacobi's equation for the Hamilton's principal functional is obtained. These results are surprisingly in great harmony with each other. They will be applied to the general relativity in the subsequent articles, especially the generalized Noether's theorem will be applied to the problem of conservation and non-conservation in curved spacetime..