Laurent Series Representation for the Open Superstring Free Energy
  Open superstrings at non-zero temperature are considered. A novel representation for the free energy (Laurent series representation) is constructed. It is shown that the Hagedorn temperature arises in this formalism as the convergence condition (specifically, the radius of convergence) of the Laurent series. 
A.A. Bytsenko, E. Elizalde, S.D. Odintsov nd S. Zerbini