Huppes, Gjalt, and Masanobu Ishikawa. “Letter to the Editorial Office: Eco-Efficiency for Sustainability: The Challenge Investigated at the 3rd Eco-Efficiency Conference, 9-11 June 2010”. Problemy Ekorozwoju, vol. 5, no. 2, July 2010, pp. 155-6, https://ph.pollub.pl/index.php/preko/article/view/4758.