We study two and three-point tree-level amplitudes of open strings.These amplitudes are reduced from higher-point correlation functions by using mostly BRST exact operators for gauge fixing.For simplicity, we focus on an open string tachyon.
The two-point amplitude of open string tachyons is reduced from a three-point function of two tachyon vertex operators and one mostly BRST exact Soy Protein Powder operator.Similarly the three-point amplitude is from a four-point function of three tachyon vertex operators and one mostly BRST exact operator.One can also obtain the two-point amplitude from the four-point function of two tachyon vertex operators and two mostly BRST exact operators.
In these derivation from four-point functions, moduli integrals are significant.We discuss the overall signs of amplitudes Water bottles which are indefinite in this formalism.