Callan, C.; Thorlacius, L. (1989). 〈Sigma Models and String Theory〉(PDF). Jevicki, A.; Tan, C.-I. 《Particles, Strings and Supernovae, Volumes Ⅰ and Ⅱ. Proceedings of the Theoretical Advanced Study Institute in Elementary Particle Physics, held June 6 - July 2, 1988, at Brown University, Providence, Rhode Island》 (영어). World Scientific. 795쪽. Bibcode:1989pss..conf..795C.
Callan, C.; Thorlacius, L. (1989). 〈Sigma Models and String Theory〉(PDF). Jevicki, A.; Tan, C.-I. 《Particles, Strings and Supernovae, Volumes Ⅰ and Ⅱ. Proceedings of the Theoretical Advanced Study Institute in Elementary Particle Physics, held June 6 - July 2, 1988, at Brown University, Providence, Rhode Island》 (영어). World Scientific. 795쪽. Bibcode:1989pss..conf..795C.