Bosonic String at Finite Temperature
  The Thermo Field Dynamics formalism is presented. In particular, it is applied to the two-dimensional field theory that describes a open bosonic string. The value of entropy operator is computed in various Dirichlet and Neumann boundary states. The Thermal Bogoliubov transformations, the thermal vacuum and the thermal Fock space are computed explicitely for the case of SU(1,1) thermal algebra. 
W. P. de Souza