A geometric classification of the path components of the space of locally stable maps $S^3\to \mathbb{R}^4$

Abstract

Locally stable maps $S^3\to\mathbb{R}^4$ are classified up to homotopy through locally stable maps. The equivalence class of a map $f$ is determined by three invariants: the isotopy class $σ(f)$ of its framed singularity link, the generalized normal degree $ν(f)$, and the algebraic number of cusps $κ(f)$ of any extension of $f$ to a locally stable map of the $4$-disk into $\mathbb{R}^5$. Relations between the invariants are described, and it is proved that for any $σ$, $ν$, and $κ$ which satisfy these relations, there exists a map $f:S^3\to\mathbb{R}^4$ with $σ(f)=σ$, $ν(f)=ν$, and $κ(f)=κ$. It follows in particular that every framed link in $S^3$ is the singularity set of some locally stable map into $\mathbb{R}^4$.