1、Y.Nambu,“Dual Model Of Hadrons”, EFI-70-07 Feb. 1970, 14pp.
2、Z. Koba and H.B. Nielsen, “Reaction Amplitude for n-mesons. A Generalization of the Veneziano-Bardakci-Ruegg-Virasoro Model”, Nucl. Phys. B10(1969)633-655
3、Z. Koba and H.B. Nielsen, Nucl. Phys. B12(1969)517