Don Nelson Page
THE OBSERVABLE AND THE NON-OBSERVABLE
PARITY OPERATOR AS A TIME-DEPENDENT OBSERVABLE.
J. Werle
Time as an observable
Jonathan Oppenheim, Benni Reznik, William George Unruh
A general perspective on time observables
Bryan Wilson Roberts
The Structure of Observables
Yoshio Ohnuki, S. Kamefuchi
TRANSFORMATION OF GENERAL COVARIANT OBSERVABLE VALUES.
N. V. Mit︠s︡kevich, В. Н. Захаров