Solving all 4-point correlation functions for bosonic open string theory   in the high energy limit
  We study the implication of decoupling zero-norm states in the high-energy limit, for the 26 dimensional bosonic open string theory. Infinitely many linear relations among 4-point functions are derived algebraically, and their unique solution is found. Equivalent results are also obtained by taking the high-energy limit of Virasoro constraints, and as an independent check, we compute all 4-point functions of 3 tachyons and an arbitrary massive state by saddle-point approximation. 
Chuan-Tsung Chan, Pei-Ming Ho, Jen-Chi Lee, Shunsuke Teraguchi, Yi   Yang