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We consider the covariant and contravariant symbols of operators that are a generalization of the Wick and anti-Wick symbols introduced earlier. With the help of these symbols we investigate the spectral characteristics of operators, and construct a method of successive approximations to calculate the exponential of an operator. In an application to anti-Wick symbols this method provides a basis of one of the variants of the Feynman functional integral along trajectories in phase space.
F. A. Berezin (Tue,) studied this question.