Quantum Poincaré covariance of the two-dimensional string

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Abstract
We show that the free massive quantum string in two space-time dimensions is Poincaré covariant. This is true for both lightlike and timelike gauge choices. When the massless limit is taken, the massless string is also seen to be Poincaré covariant. The set of classical string variables which is chosen to define the quantum theory drastically affects the proof of Poincaré covariance and also the mass spectrum. We therefore suggest that other dynamical systems may exhibit similar phenomena.