Self-intersection of parametric surface using Gauss-Bonnet theorem I am trying to detect when a closed parametric surface intersect itself. My surface is described as a triplet of parametric functions $x(u,v)$, $y(u,v)$ and $z(u,v)$ where $u,v\in[0,1]$. For that purpose I'm trying to use the Gauss-Bonnet theorem. This theorem relates the integral of the Gaussian curvature over the surface to the Euler characteristic (which is an invariant of a regular surface). My questions is the following: will the Euler characteristic computed by the Gauss-Bonnet theorem change if the surface self-intersects, and if so, in what manner? Thanks

The Gauss-Bonnet theorem relates intrinsic properties of the surface, properties that are independent of how it is immersed in the ambient space. You will not be able to detect self-intersections this way. I don't have a good suggestion for how to do this detection.