Niu, Jing, Jun Wen, Xiu-Yun Yang, and Chun-Ping Chang. 2018. “Trade Openness, Political Stability and Environmental Performance: What Kind of Long-Run Relationship?”. Problemy Ekorozwoju 13 (2):57-66. https://ph.pollub.pl/index.php/preko/article/view/5013.