N -string amplitude as a trace of half-string matrices
Chan Hong-Mo, J. Bordes, Tsou Sheung Tsun, Lukas Nellen · Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D. Particles and fields · 1989
It is shown that the tree amplitude for $N$ interacting open strings can be expressed as a trace of a product of infinite-dimensional half-string matrices, in close analogy to trace factors normally used to incorporate internal symmetry.