2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/93133For time-dependent systems the wavefunction depends explicitly on time and it is not a pure state of the Hamiltonian. We construct operators for which the above wavefunction is a pure state. The method is based on the introduction of conserved quantities $Q$ and the pure states are defined by ${\hat Q}ψ=qψ$. The conserved quantities are constructed using parametrised mechanics and the Noether theorem.13 pages, plain TEX, no figuresQuantum Physicstext