1. Bruce, J.W., Gıblın, P.J., Tar F.: Families of Surfaces in Euclidean 3-space, Focal Sets, Ridges and Umbilics. University of Liverpool, Liverpool (1995, preprint)
2. Bruce, J.W., Wılkınson, T.C.: Folding maps and focal sets. In: Singularity Theory and its Applications, Part I (Coventry 1988\1989). Lecture Notes in Mathematics, vol. 1462, pp. 63–72. Springer, Berlin (1991)
3. Bruce J.W., TarıF.: Extrema of Principal Curvature and Symmetry. Proc. Edinb. Math. Soc. 39, 397–402 (1996)
4. Hou Zhong H., Li L.: A kind of rectilinear congruences in the Minkowski 3-Space. J. Math. Res. Expos. 28(4), 911–918 (2008)
5. Izumıya S., TakahashıM., TarıF.: Folding maps on spacelike and timelike surfaces and duality. Osaka J. Math. 47, 839–862 (2010)