Sutantyo, W. “Gerak Photon Dalam Medan Gravitasi Jang Kuat Disekitar Bintang Jang Mampat.”. Journal of Mathematical and Fundamental Sciences, vol. 6, no. 2, Feb. 2019, pp. 57-63, https://journals.itb.ac.id/index.php/jmfs/article/view/9729.