The Algebra of Observables of the Closed Bosonic Nambu-Goto-String: The   Presentation of the Algebra of Classical Observables for Light-Like   Energy-Momentum in 3+1 Dimensions
  The presentation of the algebra of classical observables of the closed bosonic Nambu-Goto-String in 3+1 dimensions is given for the massless case $P^2=0,P\neq0$ in the relevant standard frame up to and including the second stratum. The elements and the relations of this algebra established so far are presented in the form of irreducible multiplets of the stabilizer group $E_2$ of the light-like vector $P$ in its standard form. 
Imke Schneider