Canceling branch points and cusps on projections of knotted surfaces in 4-space

摘要

For a knotted surface in 4-space, its generic projection into 3-space has branch points as its singularities, and its successive projection into 2-space has fold points and cusps as its singularities. In this paper, we show that for non-orientable knotted surfaces, the numbers of branch points and cusps can be minimized by isotopy.