1. V.I. Arnol’d, Indices of singular 1-forms on a manifold with boundary, convolution of invariants of reflection groups, and singular projections of smooth surfaces. Russ. Math. Surv. 34, 1–42 (1979)
2. J.W. Bruce, P.J. Giblin, Curves and Singularities, 2nd edn. (Cambridge University Press, Cambridge, 1992)
3. R. Cipolla, P. Giblin, Visual Motion of Curves and Surfaces (Cambridge University Press, Cambridge, 2000)
4. T. Gaffney, The structure of T 𝒜 ( f ) $$T\mathcal{A}(f)$$ , classification and an application to differential geometry. Part I. Proc. Symp. Pure Math. 40, 409–427 (1983)
5. P.J. Giblin, Apparent contours: an outline. Proc. R. Soc. Lond. A 356, 1087–1102 (1998)