The Nambu-Goto Theory of Closed Bosonic Strings Moving in   1+3-Dimensional Minkowski Space: The Quantum Algebra of Observables
  A relevant part of the quantum algebra of observables for the closed bosonic strings moving in 1+3-dimensional Minkowski space is presented in the form of generating relations involving still one, as yet undetermined, real free parameter. 
K. Pohlmeyer