  It is shown that if a complete set of mutually commuting operators is formed
by constants of motion, then, up to a factor that only depends on the time,
each common eigenfunction of such operators is a solution of the Schr\"odinger
equation. In particular, the operators representing the initial values of the
Cartesian coordinates of a particle are constants of motion that commute with
each other and from their common eigenfunction one readily obtains the Green
function.
