Claude, D. M. ., N. . Ibrahim, J. E. . Fantcho, and T. N. U. . Ndesil. “The Characteristics and Spatial Modelling of Informal Economies”. Asian Journal of Economic Modelling, vol. 8, no. 3, July 2020, pp. 165-88, doi:10.18488/journal.8.2020.83.165.188.