Abstract
Given a *-algebra \(\mathfrak{A}\) and a linear, nonpositive definite, functional on it, the GNS construction yields a representation of \(\mathfrak{A}\) in an indefinite metric space. We show that, if the functional satisfies a suitable condition, expressed in terms of a conditional expectation on \(\mathfrak{A}\), then the representation space is a Krein space. This provides an abstract setup for the description of a massless quantum field theory.
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