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Rudolf Seiler, Dietrich A. Uhlenbrock, and David N. Williams, "On the bound states of the massless free field in two space-time dimensions", Nucl. Phys. B116, 133–140 (1976).
Copyright (1976) © by Elsevier Science B.V.
DOI: 10.1016/0550-3213(76)90316-3