The area of a region of the phase space diagram at timeis given in terms of the area at some earlier timeby

Differentiating this expression gives

or

is a solution curve of the systemsothen for a transformation

The second bracket equals 0 for Hamiltonian systems hence

Obviously ifis constant then the system is Hamiltonian in the nnew coordinatesbut ifwheneverdoes thenfor every Hamiltonian system but given a pointin phase space we may construct a functionsuch thatandand another function such thatandat that point thus we must have thator thatis constant.