Topological discretization of bosonic strings
  We apply the method of topological quantization to obtain the bosonic string topological spectrum propagating on a flat background. We define the classical configuration of the system, and construct the corresponding principal fiber bundle (pfb) that uniquely represents it. The topological spectrum is defined through the characteristic class of the pfb. We find explicit expressions for the topological spectrum for particular configurations of the bosonic strings on a Minkowski background and show that they lead to a discretization of the total energy of the system. 
Gustavo Arciniega, Francisco Nettel, Leonardo Pati\~no, Hernando   Quevedo
math-ph gr-qc hep-th math.MP