Oliveira, D., and I. Mendes. “Green Taxation on Competitiveness: The Effect of the ISP Tax on the Portuguese Retail Sector of Road Fuels”. International Journal of Energy Economics and Policy, vol. 7, no. 1, Jan. 2017, pp. 178-84, https://www.econjournals.com/index.php/ijeep/article/view/3652.