1. Pohlmeyer, K.: A group theoretical approach to the quantization of the free relativistic closed string. Phys. Lett.119B, 100 (1982)
2. Pohlmeyer, K.: An approach towards the quantization of the relativistic closed string based upon symmetries. Seminaires de Meudon, Springer Lecture Notes in Physics, Vol.226, 159. Berlin, Heidelberg, New York: Springer 1985
3. Nambu, J.: Lectures at the Copenhagen summer symposium 1970 (unpublished);
4. Goto, T.: Relativistic quantum mechanics of one-dimensional mechanical continuum and subsidiary condition of dual resonance model. Progr. Theor. Phys.46, 1560 (1971)
5. Rehren, K.-H.: Freiburg university thesis, 1984, Physics Faculty, THEP 84/8 (in German)