Two- and three-loop amplitudes in boson string theory
Belavin A. A., Knizhnik V. G., Morozov A. Yu., Perelomov A. M.
Explicit expressions are derived for the twoand three-loop vacuum amplitudes in the theory of oriented closed boson strings with D ~ 26 in terms of the theta constants. The space of moduli is parametrized by means of period matrices.