Hameed Hamz, Sattar, and Fatima Mizheir Majhool. “Strongly N-Proper Functions”. Journal of Al-Qadisiyah for Computer Science and Mathematics, vol. 3, no. 2, Oct. 2017, pp. 183-99, https://jqcsm.qu.edu.iq/index.php/journalcm/article/view/288.