K. Pohlmeyer, The Nambu-Goto theory of closed bosonic strings moving in 1+3-dimensional Minkowski space: the quantum algebra of observables, ANN PHYSIK, 8(1), 1999, pp. 19-50
A relevant part of the quantum algebra of observables for the closed bosoni
c strings moving in I + 3-dimensional Minkowski space is presented in the f
orm of generating relations involving still one, as yet undetermined, real
free parameter.