Siregar, Naufal Ramadhan Al Akhwal, et al. “Pemodelan Harga Minyak Mentah Dunia Berdasarkan Efek Pandemi Covid-19 Dengan Estimator Penalized Spline”. MUST: Journal of Mathematics Education, Science and Technology, vol. 7, no. 2, Dec. 2022, pp. 152-66, doi:10.30651/must.v7i2.16377.