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 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 which are indefinite in this formalism.

我们研究开放弦的二点和三点树级振幅。通过主要使用 BRST 精确算子进行规范固定，这些幅度从较高点的相关函数中减少。为简单起见，我们专注于开放弦速子。开放弦速子的两点振幅从两个速子顶点算子和一个主要是 BRST 精确算子的三点函数减少。类似地，三点幅度来自三个快子顶点算子和一个主要是 BRST 精确算子的四点函数。还可以从两个快子顶点算子和两个主要是 BRST 精确算子的四点函数中获得两点幅度。在这些四点函数的推导中，模积分很重要。我们讨论在这种形式主义中不确定的振幅的整体符号。