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A general formulation for discrete-time quantum mechanics, based on Feynman's
method in ordinary quantum mechanics, is presented. It is shown that the
ambiguities present in ordinary quantum mechanics (due to noncommutativity of
the operators), are no longer present here. Then the criteria for the unitarity
of the evolution operator are examined. It is shown that the unitarity of the
evolution operator puts restrictions on the form of the action, and also
implies the existence of a solution for the classical
initial-value problem.
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