Алі, Аль-газу. “Definition of Scientific Based Approaches to the Classification of Costs of Air Enterprise”. Scientific Bulletin of Ivano-Frankivsk National Technical University of Oil and Gas (Series: Economics and Management in the Oil and Gas Industry), vol. 1, no. 13, May 2018, pp. 78-83, https://eung.nung.edu.ua/index.php/ecom/article/view/52.