Şahin, T., and M. Yilmaz. “On Singularities of the Galilean Spherical Darboux Ruled Surface of a Space Curve in $G_3$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 10, Oct. 2010, pp. 1377–1387, https://umj.imath.kiev.ua/index.php/umj/article/view/2962.