Linear relations among 4-point functions in the high energy limit of   string theory
  The decoupling of zero-norm states leads to linear relations among 4-point functions in the high energy limit of string theory. Recently it was shown that the linear relations uniquely determine ratios among 4-point functions at the leading order. The purpose of this paper is to extend the validity of the same approach to the next-to-leading order and higher orders. 
Pei-Ming Ho, Xue-Yan Lin