  Operators play a substantial role in mathematical formalism of quantum
mechanics. However, explicit forms of the operators are usually postulated,
based on the intuitive assumptions. In this study, variational principle was
applied to the basic equation for expectation value to vary a generalized form
of operator while keeping psi-function invariable. A restriction of being
expectation value invariable, allowed one to derive all possible forms of the
operators corresponding to the conserved physical entities. As a result, it was
found that only three distinctive forms of the conserved operators are
possible, tentatively assigned to be angular momentum-like, momentum-like and
total energy-like operators. Surprisingly, all operators included constant, the
same one for each of the operators, therefore, making operators the quantum
ones. Absence of the quantization in original assumptions suggests that quantum
character of the operators and, therefore, the physical entities, is a direct
consequence of the existence of the conservation laws.
