We derive a criterion that is convenient for applications and exactly
characterizes the test function space on which the operator realization of a
given series of Wick powers of a free field is possible. The suggested
derivation does not use the assumption that the metric of the state space is
positive and can therefore be used in a gauge theory. It is based on the
systematic use of the analytic properties of the Hilbert majorant of the
indefinite metric and on the application of a suitable theorem on the
unconditional convergence of series of boundary values of analytic functions.