YOON, D. W. On the second Gaussian curvature of ruled surfaces in Euclidean $3$-space. Tamkang Journal of Mathematics, [S. l.], v. 37, n. 3, p. 221-226, 2006. DOI: 10.5556/j.tkjm.37.2006.167. Disponível em: https://journals.math.tku.edu.tw/index.php/TKJM/article/view/167. Acesso em: 25 jul. 2021.