§ 参考文献
[1] H.Kawai,D.Lewellen,H.Tye,“A Relation Between Tree Amplitudes of Closed and Open Strings”,Nucl.Phys.B 269(1986)1-23。
[2] Z.Bern,J.Carrasco,and H.Johansson,“New Relations for Gauge-Theory Amplitudes”,Phys.Rev.D78(2008)085011。
[3] G.Lin,G.Yang,“Double copy for tree-level form factors I:foundations”,arXiv:2211.01386。