A Kuratowski-Type Classification of Critical Complexes for the 3-Sphere

Abstract

We give a Kuratowski-type classification of a graph-defined class of minimal piecewise-linear obstructions to embeddability in the 3-sphere. A finite simplicial complex \(X\) is called critical for \(S^3\) if \(|X|\) does not embed in \(S^3\), whereas deleting the open star of any simplex in the second barycentric subdivision of \(X\) yields a polyhedron embeddable in \(S^3\). The main theorem completely classifies critical complexes of the form \((G\times S^1)\cup H\), where \(G\) and \(H\) are graphs and \(H\) is attached along vertices of the branch set of \(G\times S^1\). We prove that there are exactly seven such complexes up to homeomorphism: two \(K_4\)-type complexes, four \(\Theta_4\)-type complexes, and one \(K_{2,3}\)-type complex. The proof is combinatorial in nature. By collapsing the \(S^1\)-factor of \(G\times S^1\), we associate to \(X\) a reduction graph \(\widehat X=G\cup H\). Criticality implies that \(H\) is a forest, \(G\) is planar, and \(\widehat X\) is inclusion-minimal non-planar. Kuratowski's theorem therefore reduces the classification to the cases \(K_5\) and \(K_{3,3}\). A finite analysis of forest attachments, together with a face-incidence criterion for embeddability, leaves precisely the seven models listed above. We also prove that every non-embeddable regular multibranched surface in \(S^3\) contains a critical subcomplex of the form \(M\cup H\), where \(M\) is a regular multibranched surface and \(H\) is a graph.