P. Biolcheva and Vasil Bozev, “The Progress of Industry 4.0 in the Conditions of a Pandemic - Bulgarian Case in Natural Gas Sector”, jstard, vol. 4, no. 1, pp. 1–8, Jun. 2022, Accessed: Aug. 12, 2026. [Online]. Available: https://journal.uthm.edu.my/index.php/jstard/article/view/11349