1. V. I. Arnold, S. M. Gusein-Zade, and A. N. Varchenko, Singularities of Differentiable Maps. Vol. I: The Classification of Critical Points, Caustics and Wave Fronts., Monographs in Mathematics, vol. 82, Birkhäuser, Boston, 1985.
2. W. Ebeling and S. M. Gusein-Zade, “On the index of a vector field at an isolated singularity,” in: The Arnoldfest (Toronto, ON, 1997), Fields Inst. Commun., vol. 24, Amer. Math. Soc., Providence, RI, 1999, 141–152.
3. W. Ebeling and S. M. Gusein-Zade, “Indices of 1-forms on an isolated complete intersection singularity,” Mosc. Math. J., 3:2 (2003), 439–455.
4. W. Ebeling and S. M. Gusein-Zade, “Indices of vector fields or 1-forms and characteristic numbers,” Bull. London Math. Soc., 37:5 (2005), 747–754.
5. W. Ebeling and S. M. Gusein-Zade, “Radial index and Euler obstruction of a 1-form on a singular variety,” Geom. Dedicata, 113 (2005), 231–241.