Fujimoto, T., C. Herrero, R. R. Ranade, J. A. Silva, and A. Villar. “A Complete Characterization of Economies With the Nonsubstitution Property”. Economic Issues, vol. 8, no. 2, Sept. 2003, pp. 63-70, http://economicissues.org.uk/index.php/EI_OJS/article/view/101.