We study the flat geometry of the least degenerate singularity of a singular surface in $\mathbb R^4$, the $I_{ 1}$ singularity parametrised by $(x,y)\mapsto(x,xy,y^{ 2},y^{ 3})$. This singularity appears generically when projecting a regular surface in $\mathbb R^5$ orthogonally to $\mathbb R^4$ along a tangent direction. We obtain a generic normal form for $I_1$ invariant under diffeomorphisms in the source and isometries in the target. We then consider the contact with hyperplanes by classifying submersions which preserve the image of $I_1$. The main tool is the study of the singularities of the height function.
Categories: math.DG