Reduction of open string amplitudes by mostly BRST exact operators

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