Adiabatic invariant in light of Hamilton's principle

Adiabatic invariants are specific physical quantities which do not change appreciably even after a very long time when the Hamiltonian of a mechanical system undergoes a slow change in time. Existing proofs of this nice feature range from sophistication, and typically resort to a sort of averaging principle using Hamilton's equations of motion. We show that a much simpler argument based directly on Hamilton's principle per se is possible. Furthermore, this approach readily reveals an interesting local recurrent property of the adiabatic invariants that is rarely emphasized in the existing literature. We also show how our simpler approach can be easily generalized to derive the time dependence of the adiabatic invariant. ? 2020 The Physical Society of the Republic of China (Taiwan)