The Nambu-Goto Theory of Closed Bosonic Strings Moving in   1+3-Dimensional Minkowski Space: The Construction of the Quantum Algebra of   Observables up to Degree Five
  The quantum algebra of observables postulated in hep-th/9805057 is constructed up to degree five. All independent relations of degree four are given; they involve three as yet undetermined parameters. Definitions and symbols are used as introduced in the above-mentioned article. 
G. Handrich and C. Nowak