BezkorovainaL. L., and VashpanovaT. Y. “$A$-Deformations of a Surface With Stationary Lengths of $LGT$-Lines”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 7, July 2010, pp. 878–884, https://umj.imath.kiev.ua/index.php/umj/article/view/2921.