Isa Shuaibu, M., A. Rafindadi Sanusi, and M. Bala Adamu. “Substitutability of Currency in Nigeria: A Non-Linear Approach”. Gusau International Journal of Management and Social Sciences, vol. 4, no. 3, Oct. 2021, p. 18, https://gijmss.com.ng/index.php/gijmss/article/view/83.